140 lines
5.8 KiB
Markdown
140 lines
5.8 KiB
Markdown
# Chapter 8. Functions
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**Function** = a block of JS code that's defined once but may be invoked multiple times.
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* *argument*: values provided by during invocation as function's parameters.
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* *return value*: computed by function using argument values
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* *invocation context*: value of `this` keyword for each invocation
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Utilization of functions:
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* **method of object**: if function is assigned to a property of an obj.
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* When a function is invoked on an object (e.g. `obj.method()`), this object become invocation context (aka `this`) for the function
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* **Constructor**: function designed to initialize a new obj.
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Function as **first class citizen** (object):
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* In JS, functions are obj. Hence, can be manipulated by programs
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* We can set properties on functions and invoke methods on them (i.e. pass function as parameter to method)
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JS function definitions can be nested within other functions
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## 8.1 Defining Functions
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### 8.1.1 Function Declarations
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**Function declaration (函数声明)** = `function` keyword + Identifier as function name + `(param1, param2, ...)` + `{JS statements as function body}`
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* `return` causes function to stop executing and return computed value to caller
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* if body has no `return`, value of function is `undefined`
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```js
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// Print the name and value of each property of o. Return undefined.
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function printprops(o) {
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for(let p in o) {
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console.log(`${p}: ${o[p]}\n`);
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}
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}
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// Compute the distance between Cartesian points (x1,y1) and (x2,y2).
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function distance(x1, y1, x2, y2) {
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let dx = x2 - x1;
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let dy = y2 - y1;
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return Math.sqrt(dx*dx + dy*dy);
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}
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// A recursive function (one that calls itself) that computes factorials
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// Recall that x! is the product of x and all positive integers less than it.
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function factorial(x) {
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if (x <= 1) return 1;
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return x * factorial(x-1);
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}
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```
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* Name of function (in function declaration) becomes a variable, whose value is function itself.
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* function declaration statements are "hoisted" (level up) to top of enclosing block.
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* All function in a JS block will be defined before JS interpreter start execution
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### 8.1.2 Function Expressions
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Multiple Function Expression e.g.
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```js
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// This function expression defines a function that squares its argument.
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// Note that we assign it to a variable
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const square = function(x) { return x*x; };
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// Function expressions can include names, which is useful for recursion.
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const f = function fact(x) { if (x <= 1) return 1; else return x*fact(x-1); };
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// Function expressions can also be used as arguments to other functions:
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[3,2,1].sort(function(a,b) { return a-b; });
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// Function expressions are sometimes defined and immediately invoked:
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let tensquared = (function(x) {return x*x;}(10));
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```
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**Function Expression (FE 函数表达式)**:
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* FE appear within context of a larger expression, or within statement
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* name of function in FE is *optional*. (e.g. 1st FE e.g. has no function name)
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* FE/FD declare variable:
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* How FD use variable: (follow e.g. in 8.1.1 `function factorial(x)`) declares a variable and assigns a function obj to it.
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* How FE use variable: developer can decide whether assign the newly defined function obj to a const or var, so we can refer to it mult-times later. (e.g. 3rd & 5th FE e.g. does not assign function obj to obj, and directly use it)
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* Good practice: assign FE to `const` to protect function obj.
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**FD vs. FE**:
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* Function defined by FD: the func obj are created before the script get executed (i.e. hoisted), so we can call these functions from code that appears above FD.
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* Functions defined by FE **DO NOT EXIST** until FE are evaluated.
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* To invoke a function (either defined using FE/FD), JS must can refer to it, function defined by FE cannot be referred until it's assigned to a variable
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### 8.1.3 Arrow Functions
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In ES6, **Arrow Function** provide more compact function syntax. **Use arrow => to separate function parameters from function body**
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Syntax of arrow function:
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* *general form*: comma-separated list of params in parentheses, followed by `=>` arrow, followed by function body in curly braces
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```js
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const func_var = (param1, param2) => { return param1 + param2; };
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```
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* *compact form (single return)*: if function body is a single `return` statement, omit `return`, semicolon, and curly braces
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```js
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const func_var = (param1, param2) => param1 + param2;
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```
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* *compact form (1 param)*: if arrow func has only 1 param, omit `()`
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```js
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const func_var = param => param*param + 2*param;
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```
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* *compact form (no param)*: if arrow func has no param, `()` must be there
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```js
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const constantFunc = () => 42;
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```
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Additional Syntax rules:
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* **No newline btw `(param)` and `arrow`**: it will create valid statement with other meaning (e.g. `const polynomial = x`)
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* **{} must needed in body for single return statement**: it will avoid syntactic ambiguity:
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```js
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const f = x => { return { value: x }; }; // Good: f() returns an object
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const g = x => ({ value: x }); // Good: g() returns an object
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const h = x => { value: x }; // Bad: h() returns nothing
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const i = x => { v: x, w: x }; // Bad: Syntax Error
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```
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#### Where to use arrow function?
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It's like lambda function in Python, it's ideal to pass arrow function to another function
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```js
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// Make a copy of an array with null elements removed.
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let filtered = [1,null,2,3].filter(x => x !== null); // filtered == [1,2,3]
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```
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#### Arrow Func vs other Funcs
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* Arrow func inherit value of `this` from the environment (where it's defined)
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* Arrow func do not have a `prototype` property, hence it cannot be used as constructor for new classes.
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### 8.1.4 Nested Functions
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```js
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function hypotenuse(a, b) {
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function square(x) { return x*x; }
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return Math.sqrt(square(a) + square(b));
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}
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```
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Scoping rule of nested function: enclosure function can access param and var of the functions they are nested within (i.e. inner function know outer function's param) |