577 lines
22 KiB
Markdown
577 lines
22 KiB
Markdown
# Chapter 6. Objects
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## 6.1 Introduction to Objects
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* An object is an unordererd collection of properties (each is a name/value pair)
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* JS object can inherit properties from another object (aka "prototype")
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* JS objects are dynamic; i.e. properties can be added/removed
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* Any value in JS is object except string, number, Symbol, `true`/`false`, `null`/`undefined`
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* Objects are mutable, and manupulated by reference rather than value.
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* e.g. `let y=x` means `y` holds a reference to the same obj, not a copy of that obj.
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* Common operations on obj: create, set, query, delete, test, and enumerate
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* Property has name & value, but no obj has two properties with the same name (that's why we use Symbol)
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* JS use *own property* to refer to non-inherited properties.
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* Each property has 3 property **attributes**:
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* **writable**: whether value of property can be set
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* **enumerable**: whether the property name is returned by for/in loop
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* **configurable**: whether the property can be deleted and its attributes can be altered
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## 6.2 Creating Objects
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Creating obj, 4 methods:
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1. using object literal
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2. using keyword `new`
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3. using `Object.create()` function
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### 6.2.1 Object Literals
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**Object literal** in simplest form:
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* comma-separated list of colon-separated `name:value` pairs, enclosed within `{}`.
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* property *name*: JS identifier or string
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* property *value*: JS expression
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```js
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let empty = {}; // An object with no properties
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let point = { x: 0, y: 0 }; // Two numeric properties
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let p2 = { x: point.x, y: point.y+1 }; // More complex values
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let book = {
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"main title": "JavaScript", // These property names include spaces,
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"sub-title": "The Definitive Guide", // and hyphens, so use string literals.
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for: "all audiences", // for is reserved, but no quotes.
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author: { // The value of this property is
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firstname: "David", // itself an object.
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surname: "Flanagan"
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}
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};
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```
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When object literal works:
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* Object literal creates & initializes a new & distinct obj every time it's evaluated.
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* In loop body, a new obj can be created repeatedly.
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### 6.2.2 Creating Objects with new
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* **`new` operator**
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* creates & initialize a new object
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* syntax: `new` followed by function invocation as **constructor**
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```js
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let o = new Object(); // Create an empty object: same as {}.
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let a = new Array(); // Create an empty array: same as [].
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let d = new Date(); // Create a Date object representing the current time
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let r = new Map(); // Create a Map object for key/value mapping
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```
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### 6.2.3 Prototypes
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Almost all JS obj has a prototype associate with it:
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* All objs created using **object literal** (shown in 6.2.1) are associated with the same prototype obj, referred by `Object.prototype`
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* objs created using `new` (invoking constructor) use *value of constructor function's `prototype` property* as prototype
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* `new Object()` inherits from `Object.prototype`
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* `new Array()` inherits from `Array.prototype`
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* Only few objects have `prototype` property, they are used to define `prototypes` for all other objs.
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### 6.2.4 Object.create()
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3 Methods below demonstrated ability to create a new obj with an arbitrary prototype:
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* Create new obj w/ defined prototype using **`Object.create()`**
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```js
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let o1 = Object.create({x: 1, y: 2}); // o1 inherits properties x and y.
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o1.x + o1.y // => 3
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```
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* Create new obj w/o prototype by parsing `null`
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* Created obj inherit no property or method (e.g. `toString()`)
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```js
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let o2 = Object.create(null); // o2 inherits no props or methods.
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```
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* Create ordinary new empty obj using **`Object.prototype`** (like obj returned by `{}` or `Object()`)
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```js
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let o3 = Object.create(Object.prototype); // o3 is like {} or new Object().
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```
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#### Use created object to guard unintended modification
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* Q: How to guard against unintended modification of an obj by a function (from other library)?
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* A: Instead of passing the obj directly to the function, pass an obj that inherit from it. So writing property do not affect original value. (like passing a read-only)
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```js
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let o = { x: "don't change this value" };
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library.function(Object.create(o)); // Guard against accidental modifications
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```
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## 6.3 Querying and Setting Properties
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Obtain value of property:
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* using dot (`.`): RHS of dot should be simple identifier (not string) of property
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* using square bracket (`[]`): value within `[]` should be an expression that evalutes to a string (or sth can convert to string) that contains property name
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```js
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let author = book.author; // Get the "author" property of the book.
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let name = author.surname; // Get the "surname" property of the author.
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let title = book["main title"]; // Get the "main title" property of the book.
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```
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Create/Set a property:
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* Query property, and place it on LHS
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```js
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book.edition = 7; // Create an "edition" property of book.
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book["main title"] = "ECMAScript"; // Change the "main title" property.
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```
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### 6.3.1 Objects As Associative Arrays
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```js
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object.property // C like structure access
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object["property"] // associative array
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```
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JS objects are **Associative Arrays** (e.g. hash or map or dictionary)
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* In strong typed language (e.g. C/C++), obj's property are defined. While, JS program can **create any number of properties in any object in runtime**
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* `.` operator requires name of the property as identifier, which may be unknown in code.
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* `[]` operator allow access properties dynamically
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Following code shows calculate portfolio value in runtime via associative arrays
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```js
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function computeValue(portfolio) {
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let total = 0.0;
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for(let stock in portfolio) { // For each stock in the portfolio:
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let shares = portfolio[stock]; // get the number of shares
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let price = getQuote(stock); // look up share price
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total += shares * price; // add stock value to total value
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}
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return total; // Return total value.
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}
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```
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### 6.3.2 Inheritance
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JS obj have a set of "own properties", and they also inherit properties from prototype chain.
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Read properties:
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* If a property cannot be found in a JS obj, it will search one by one (bottom to top, from child to parent) through prototype chain
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```js
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let o = {}; // o inherits object methods from Object.prototype
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o.x = 1; // and it now has an own property x.
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let p = Object.create(o); // p inherits properties from o and Object.prototype
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p.y = 2; // and has an own property y.
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let q = Object.create(p); // q inherits properties from p, o, and...
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q.z = 3; // ...Object.prototype and has an own property z.
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let f = q.toString(); // toString is inherited from Object.prototype
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q.x + q.y // => 3; x and y are inherited from o and p
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```
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Write (assign) properties:
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* check prototype chain only to verify whether read-only.
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* If inherited property `x` is read-only, assignment is not allowed
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* If assignment is allowed, the property is created/set within the current object, and do not modify prototype chain
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* Only exception: if `o` inherits property `x`, and that property is an accessor property with a setter method, then the setter method is called rather than creating a new property `x` within `o`.
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```js
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let unitcircle = { r: 1 }; // An object to inherit from
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let c = Object.create(unitcircle); // c inherits the property r
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c.x = 1; c.y = 1; // c defines two properties of its own
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c.r = 2; // c overrides its inherited property
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unitcircle.r // => 1: the prototype is not affected
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```
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### 6.3.3 Property Access Errors
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Errors during accessing property:
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* It's not error to query a property/object that does not exist. Return `undefined`
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* It's error to query property of an non-existent object. Return `TypeError`
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Query non-exist object
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```js
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book.subtitle // => undefined: property doesn't exist
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```
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Query property of an non-exist object
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```js
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let len = book.subtitle.length; // !TypeError: undefined doesn't have length
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```
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Method to guard against this problem type:
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* Method 1: verbose and explicit
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```js
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let surname = undefined;
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if (book) {
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if (book.author) {
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surname = book.author.surname;
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}
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}
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```
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* Method 2: A concise and idiomatic alternative to get surname or null or undefined
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* Check Chap4.10.1 for short-circuiting behavior of && operator
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```js
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surname = book && book.author && book.author.surname;
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```
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* Method 3: Rewrite method 2 using `?.`
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```js
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let surname = book?.author?.surname;
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```
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Lists of tips:
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* Attempting to set property on `null` or `undefined` causes a `TypeError`
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* Attempting to set property may fail due to
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* Some properties are read-only
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* Some objects don't allow adding new properties
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* Error from property assignment:
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* In strict mode (Chap 5.6.3), a TypeError is thown whenever an attempt to set a property fails.
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* Outsie strict mode, silent when fail
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3 circumstances when failed to set a property `p` of obj `o`:
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1. `o` has an own property `p` that is read-only
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2. `o` has an inherited property `p` that is read-only: No way to overwrite this property
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3. `o` does not have an own property `p`; `o` does not inherit a property `p` with a setter method, and `o`’s extensible attribute (see §14.2) is `false`.
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1. Since `p` does not already exist in `o`, and if there is no setter method to call, then `p` must be added to `o`.
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2. But if `o` is not extensible, then no new properties can be defined on it.
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## 6.4 Deleting Properties
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`delte` operator removes a property from an obj. Its operand is *property access expression*
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`delete` & prototypes:
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* only delete own properties, cannot delete inherited ones (from prototypes).
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* deleting inherited properties need to it on prototype, which has affect all children obj.
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`delete` return true if:
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* delete succeeded
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* when delete had no effect (e.g.
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* on inherited properties
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* non-exist properties
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* when used with an expression that's not a property access expression
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```js
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let o = {x: 1}; // o has own property x and inherits property toString
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delete o.x // => true: deletes property x
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delete o.x // => true: does nothing (x doesn't exist) but true anyway
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delete o.toString // => true: does nothing (toString isn't o's own property)
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delete 1 // => true: nonsense, but true anyway
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```
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`delete` fails if:
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* trying to rm properties that have a `configurable` attribute of `false`.
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* trying to delete non-configurable properties of built-in objects
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* properties of the global object created by variable declaration and function declaration
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`delete`'s failure in strict & non-strict mode:
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* In strict mode: causes TypeError
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* In non-strict mode: evaluate to `false` e.g. shown below
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```js
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// In strict mode, all these deletions throw TypeError instead of returning false
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delete Object.prototype // => false: property is non-configurable
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var x = 1; // Declare a global variable
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delete globalThis.x // => false: can't delete this property
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function f() {} // Declare a global function
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delete globalThis.f // => false: can't delete this property either
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```
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`delete` configurable properties of global object:
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* In non-strict mode, via directly delete property name
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```js
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globalThis.x = 1; // Create a configurable global property (no let or var)
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delete x // => true: this property can be deleted
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```
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* In strict mode, method above will create **SyntaxError**. Hence need to be explicit:
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```js
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delete x; // SyntaxError in strict mode
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delete globalThis.x; // This works
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```
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## 6.5 Testing Properties
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JS obj = sets of properties. We want to test membership in the set using:
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* `in` operator
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* `hasOwnProperty()` method
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* `propertyIsEnumerable()` method
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* querying property
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`in` operator return true if object has an own property or an inherited property **by that name**:
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```js
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let o = { x: 1 };
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"x" in o // => true: o has an own property "x"
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"y" in o // => false: o doesn't have a property "y"
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"toString" in o // => true: o inherits a toString property
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```
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`hasOwnProperty()` method of an obj return:
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* `true` if obj has an own property with the given name
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* `false` for inherited properties
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```js
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let o = { x: 1 };
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o.hasOwnProperty("x") // => true: o has an own property x
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o.hasOwnProperty("y") // => false: o doesn't have a property y
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o.hasOwnProperty("toString") // => false: toString is an inherited property
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```
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`propertyIsEnumerable()` refines `hasOwnProperty()`. It returns:
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* `true` if named property is not inherited and its *enumerable* attribute is `true`
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* Properties created by normal JS code are enumerable unless specified
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```js
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let o = { x: 1 };
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o.propertyIsEnumerable("x") // => true: o has an own enumerable property x
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o.propertyIsEnumerable("toString") // => false: not an own property
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Object.prototype.propertyIsEnumerable("toString") // => false: not enumerable
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```
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Qeurying method is simple by using `!=` to make sure whether property is undefined
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```js
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let o = { x: 1 };
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o.x !== undefined // => true: o has a property x
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o.y !== undefined // => false: o doesn't have a property y
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o.toString !== undefined // => true: o inherits a toString property
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```
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Simple querying method vs. `in` operator:
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* `in` operator can tell whether a property does not exit (`null`) or exit but not defined (`undefined`)
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```js
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let o = { x: undefined }; // Property is explicitly set to undefined
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o.x !== undefined // => false: property exists but is undefined
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o.y !== undefined // => false: property doesn't even exist
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"x" in o // => true: the property exists
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"y" in o // => false: the property doesn't exist
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delete o.x; // Delete the property x
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"x" in o // => false: it doesn't exist anymore
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```
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## 6.6 Enumerating Properties
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We want to iterate through or obtain a list of all properties of an object using:
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* `for/in` loop.
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* Get an array of property names for an object and then loop through that array with `for/of` loop.
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**Method 1: Use `for/in` loop to enumerate properties**
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1. runs the body of the loop once for each enumerable property (own or inherited) of the specified obj.
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2. assigning the name of the property to the loop variable.
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Note:
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* Inherited built-in methods are not enumerable (e.g. "toString")
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* Properties that added by code are enumerable by default.
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```js
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let o = {x: 1, y: 2, z: 3}; // Three enumerable own properties
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o.propertyIsEnumerable("toString") // => false: not enumerable
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for(let p in o) { // Loop through the properties
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console.log(p); // Prints x, y, and z, but not toString
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}
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```
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Trick: Stop enumerating inherited properties with `for/in`, add an explicit check inside loop body
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```js
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for(let p in o) {
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if (!o.hasOwnProperty(p)) continue; // Skip inherited properties
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}
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for(let p in o) {
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if (typeof o[p] === "function") continue; // Skip all methods
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}
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```
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**Method 2: Get an array of property names for an object and then loop through that array with `for/of` loop.**
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Four ways to get an array of property names:
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* `Object.keys()`: returns an array of the names of the enumerable own properties of an object, excluding non-enumerable properties, inherited properties, properties whose name is Symbol
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* `Object.getOwnPropertyNames()`: returns same as `Ojbect.keys()` + non-enum own properties (as long as name are strings)
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* `Object.getOwnPropertySymbols()`: returns own properties whose names are Symboles (no matter they are enumerable or not)
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* `Reflect.ownKeys()`: returns all own property names, both enum or non-enum, and both string and Symbol.
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### 6.6.1 Property Enumeration Order
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Order Summary:
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1. String properties whose names are non-negative integers, numeric order from smallest to largest. i.e. arrays are enumerated in order
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2. All remaining properties with string names. In order they were added to object.
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3. Properties whose names are Symbol in order they were added to obj.
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## 6.7 Extending Objects
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Common practice in pure JS to copy properties from one obj to another:
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```js
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let target = {x: 1}, source = {y: 2, z: 3};
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for(let key of Object.keys(source)) {
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target[key] = source[key];
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}
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target // => {x: 1, y: 2, z: 3}
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```
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Various JS frameworks have developed utility function `extend()` to cover this operation, and it's standarized in ES6 as `Object.assign()`
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**`Object.assign()`**:
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* Syntax: `Object.assign(target_obj, src_obj_1, src_obj_2)`
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* Operations: Copy enumerable own properties from 2nd and subsequent args (i.e. **source object**) to 1st arg (i.e. **target object**)
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* target obj is modified & returned
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* source obj is not changed
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* Copy order follows order of arg, so 1st source object property will overwrite target obj property, while 2nd source obj property will overwrite 1st obj property
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* How copy works: using ordinary property get/set operations.
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* if source has getter method, and target has setter, these 2 methods will be invoked. But themselves won't be copied.
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Trick to use `.assign()`:
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* Given obj `o`, and source `defaults`. Directly assign will overwrite `o`'s original properties if there are same property names
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```js
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Object.assign(o, defaults); // overwrites everything in o with defaults
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```
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* Correct way of safely copy properties while keeping target's original property values: create a new object, copy the defaults into it, and then override those defaults with the properties in `o`
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```js
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o = Object.assign({}, defaults, o);
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```
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A function can be created to solve this problem via copies properties only if they are missing:
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```js
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// Like Object.assign() but doesn't override existing properties
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// (and also doesn't handle Symbol properties)
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function merge(target, ...sources) {
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for(let source of sources) {
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for(let key of Object.keys(source)) {
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if (!(key in target)) { // This is different than Object.assign()
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target[key] = source[key];
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}
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}
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}
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return target;
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}
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Object.assign({x: 1}, {x: 2, y: 2}, {y: 3, z: 4}) // => {x: 2, y: 3, z: 4}
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merge({x: 1}, {x: 2, y: 2}, {y: 3, z: 4}) // => {x: 1, y: 2, z: 4}
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```
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## 6.8 Serializing Objects
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**Object serialization** = process to convert an object's state to a string. Then later we can restore it.
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2 functions:
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* `JSON.stringify()`: serialize JS objects.
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* `JSON.parse`: restore JS objects.
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JSON stands for **"JavaScript Object Notation"**
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```js
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let o = {x: 1, y: {z: [false, null, ""]}}; // Define a test object
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let s = JSON.stringify(o); // s == '{"x":1,"y":{"z":[false,null,""]}}'
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let p = JSON.parse(s); // p == {x: 1, y: {z: [false, null, ""]}}
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```
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What can/cannot be serialized?
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* CAN be serialized: Objects, arrays, strings, finite numbers, `true`, `false`, `null`
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* `NaN`, `Infinity`, `-Infinity` serialized to `null`
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* CANNOT be serialized: Function, RegExp, Error objects, `undefined` value
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||
## 6.9 Object Methods
|
||
|
||
All JS objects (excpet explicitly created w/o prototype) inherits properties from `Object.prototype`. Hence they inherited some **primarily methods**, which are universally available.
|
||
* `hasOwnProperty()`, `propertyIsEnumerable()`
|
||
* `Object.create()`, `Object.keys()`
|
||
* `toString`
|
||
* ...
|
||
|
||
### 6.9.1 The toString() Method
|
||
|
||
* `toString()` takes no arg, and returns a string that represents the value of obj.
|
||
* Default `toString()` is not useful (e.g. shown below), each class define their own `toString()`
|
||
|
||
```js
|
||
let s = { x: 1, y: 1 }.toString(); // s evaluate to "[object Object]", without show real information
|
||
```
|
||
|
||
* We can redefine `toString()` method:
|
||
|
||
```js
|
||
let point = {
|
||
x: 1,
|
||
y: 2,
|
||
toString: function() { return `(${this.x}, ${this.y})`; }
|
||
};
|
||
|
||
String(point) // => "(1, 2)": toString() is used for string conversions
|
||
```
|
||
|
||
### 6.9.2 The toLocaleString() Method
|
||
|
||
`toLocaleString()`:
|
||
* All objs have this method
|
||
* Purpose of this method: return a localized (vs. internationalization) string representation of the obj.
|
||
* default `toLocaleString()` defined by Object call `toString()` directly
|
||
* Date/Number class defin customized versions of `toLocaleString()` to format date, currency, time, etc.
|
||
* User can create their own method:
|
||
|
||
```js
|
||
let point = {
|
||
x: 1000,
|
||
y: 2000,
|
||
toString: function() { return `(${this.x}, ${this.y})`; },
|
||
toLocaleString: function() {
|
||
return `(${this.x.toLocaleString()}, ${this.y.toLocaleString()})`;
|
||
}
|
||
};
|
||
point.toString() // => "(1000, 2000)"
|
||
point.toLocaleString() // => "(1,000, 2,000)": note thousands separators
|
||
```
|
||
|
||
### 6.9.3 The valueOf() Method
|
||
|
||
`valueOf()`:
|
||
* is called when JS needs to convert an obj to some non-string primitive type (e.g. number)
|
||
* Many built-in class has its own `valueOf()`
|
||
* Date class define its `valueOf()` to convert dates to number, so can perform comparison
|
||
|
||
### 6.9.4 The toJSON() Method
|
||
|
||
`toJSON()` method:
|
||
* didn't get defined in `Object.prototype`;
|
||
* `JSON.stringify()` looks for & invoke it on any object that will be serialized
|
||
|
||
```js
|
||
let point = {
|
||
x: 1,
|
||
y: 2,
|
||
toString: function() { return `(${this.x}, ${this.y})`; },
|
||
toJSON: function() { return this.toString(); }
|
||
};
|
||
JSON.stringify([point]) // => '["(1, 2)"]'
|
||
```
|
||
|
||
## 6.10 Extended Object Literal Syntax
|
||
|
||
|
||
|
||
### 6.10.1 Shorthand Properties
|
||
|
||
### 6.10.2 Computed Property Names
|
||
|
||
### 6.10.3 Symbols as Property Names
|
||
|
||
### 6.10.4 Spread Operator
|
||
|
||
### 6.10.5 Shorthand Methods
|
||
|
||
### 6.10.6 Property Getters and Setters
|
||
|
||
* All obj properties mentioned above are **data properties**. JS also supports **accessor properties** (
|
||
* A accessor property does not have a value, but has 1 or 2 accessor methods: **getter** or **setter**)
|
||
|
||
How accessor property works:
|
||
* When JS program queries value of this accessor property, JS invoke getter method.
|
||
* When JS program sets value of the accessor property, JS invokes setter method, and passing value.
|
||
|
||
R/W property?:
|
||
* If a accessor property has both getter & setter, it's a **read/write property**.
|
||
* If it only has a getter method, it's a **read-only property**.
|
||
* If it only has a setter method, it's a **write-only property**. (not possible with data properties)
|
||
* Read it return `undefined`
|
||
|
||
Accessor property syntax using an extension to object literal syntax:
|
||
```js
|
||
let o = {
|
||
// An ordinary data property
|
||
dataProp: value,
|
||
|
||
// An accessor property defined as a pair of functions.
|
||
get accessorProp() { return this.dataProp; },
|
||
set accessorProp(value) { this.dataProp = value; }
|
||
};
|
||
```
|