# Chapter 6. Objects ## 6.1 Introduction to Objects * An object is an unordererd collection of properties (each is a name/value pair) * JS object can inherit properties from another object (aka "prototype") * JS objects are dynamic; i.e. properties can be added/removed * Any value in JS is object except string, number, Symbol, `true`/`false`, `null`/`undefined` * Objects are mutable, and manupulated by reference rather than value. * e.g. `let y=x` means `y` holds a reference to the same obj, not a copy of that obj. * Common operations on obj: create, set, query, delete, test, and enumerate * Property has name & value, but no obj has two properties with the same name (that's why we use Symbol) * JS use *own property* to refer to non-inherited properties. * Each property has 3 property **attributes**: * **writable**: whether value of property can be set * **enumerable**: whether the property name is returned by for/in loop * **configurable**: whether the property can be deleted and its attributes can be altered ## 6.2 Creating Objects Creating obj, 4 methods: 1. using object literal 2. using keyword `new` 3. using `Object.create()` function ### 6.2.1 Object Literals **Object literal** in simplest form: * comma-separated list of colon-separated `name:value` pairs, enclosed within `{}`. * property *name*: JS identifier or string * property *value*: JS expression ```js let empty = {}; // An object with no properties let point = { x: 0, y: 0 }; // Two numeric properties let p2 = { x: point.x, y: point.y+1 }; // More complex values let book = { "main title": "JavaScript", // These property names include spaces, "sub-title": "The Definitive Guide", // and hyphens, so use string literals. for: "all audiences", // for is reserved, but no quotes. author: { // The value of this property is firstname: "David", // itself an object. surname: "Flanagan" } }; ``` When object literal works: * Object literal creates & initializes a new & distinct obj every time it's evaluated. * In loop body, a new obj can be created repeatedly. ### 6.2.2 Creating Objects with new * **`new` operator** * creates & initialize a new object * syntax: `new` followed by function invocation as **constructor** ```js let o = new Object(); // Create an empty object: same as {}. let a = new Array(); // Create an empty array: same as []. let d = new Date(); // Create a Date object representing the current time let r = new Map(); // Create a Map object for key/value mapping ``` ### 6.2.3 Prototypes Almost all JS obj has a prototype associate with it: * All objs created using **object literal** (shown in 6.2.1) are associated with the same prototype obj, referred by `Object.prototype` * objs created using `new` (invoking constructor) use *value of constructor function's `prototype` property* as prototype * `new Object()` inherits from `Object.prototype` * `new Array()` inherits from `Array.prototype` * Only few objects have `prototype` property, they are used to define `prototypes` for all other objs. ### 6.2.4 Object.create() 3 Methods below demonstrated ability to create a new obj with an arbitrary prototype: * Create new obj w/ defined prototype using **`Object.create()`** ```js let o1 = Object.create({x: 1, y: 2}); // o1 inherits properties x and y. o1.x + o1.y // => 3 ``` * Create new obj w/o prototype by parsing `null` * Created obj inherit no property or method (e.g. `toString()`) ```js let o2 = Object.create(null); // o2 inherits no props or methods. ``` * Create ordinary new empty obj using **`Object.prototype`** (like obj returned by `{}` or `Object()`) ```js let o3 = Object.create(Object.prototype); // o3 is like {} or new Object(). ``` #### Use created object to guard unintended modification * Q: How to guard against unintended modification of an obj by a function (from other library)? * 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) ```js let o = { x: "don't change this value" }; library.function(Object.create(o)); // Guard against accidental modifications ``` ## 6.3 Querying and Setting Properties Obtain value of property: * using dot (`.`): RHS of dot should be simple identifier (not string) of property * using square bracket (`[]`): value within `[]` should be an expression that evalutes to a string (or sth can convert to string) that contains property name ```js let author = book.author; // Get the "author" property of the book. let name = author.surname; // Get the "surname" property of the author. let title = book["main title"]; // Get the "main title" property of the book. ``` Create/Set a property: * Query property, and place it on LHS ```js book.edition = 7; // Create an "edition" property of book. book["main title"] = "ECMAScript"; // Change the "main title" property. ``` ### 6.3.1 Objects As Associative Arrays ```js object.property // C like structure access object["property"] // associative array ``` JS objects are **Associative Arrays** (e.g. hash or map or dictionary) * 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** * `.` operator requires name of the property as identifier, which may be unknown in code. * `[]` operator allow access properties dynamically Following code shows calculate portfolio value in runtime via associative arrays ```js function computeValue(portfolio) { let total = 0.0; for(let stock in portfolio) { // For each stock in the portfolio: let shares = portfolio[stock]; // get the number of shares let price = getQuote(stock); // look up share price total += shares * price; // add stock value to total value } return total; // Return total value. } ``` ### 6.3.2 Inheritance ### 6.3.3 Property Access Errors ## 6.4 Deleting Properties ## 6.5 Testing Properties ## 6.6 Enumerating Properties ### 6.6.1 Property Enumeration Order ## 6.7 Extending Objects ## 6.8 Serializing Objects ## 6.9 Object Methods ### 6.9.1 The toString() Method ### 6.9.2 The toLocaleString() Method ### 6.9.3 The valueOf() Method ### 6.9.4 The toJSON() Method ## 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