1. Defined Functions
A defined way
1. The absolute value function
function abs(x){
if(x>=0){
return x;
}else{
return -x;
}
}
Once executed on behalf return to the function ends and returns the result!
If no return, executing the function will return a result, the result is undefined
Second way is defined
var abs = function(x){
if(x>=0){
return x;
}else{
return -x;
}
}
function (x) {....} This is an anonymous function. However, you can assign the result to abs, you can call the function by abs!
Second way and a way equivalent!
call function
abs(10) //10
abs(-10) //10
Parameter Problem: javaScript can pass any parameters, the parameters may not pass ~
var abs = function(x){
//手动抛出异常来判断
if (typeof x!== 'number') {
throw 'Not a Number';
}
if(x>=0){
return x;
}else{
return -x;
}
}
arguments
argumentsIs a keyword complimentary JS;
Representatives passed in all parameters, is an array!
var abs = function(x){
console.log("x=>"+x);
for (var i = 0; i<arguments.length;i++){
console.log(arguments[i]);
}
if(x>=0){
return x;
}else{
return -x;
}
}
Question: arguments contains all the parameters, we sometimes want to use the extra parameters to append operation. Need to exclude existing parameters -
rest
before:
if (arguments.length>2){
for (var i = 2; i<arguments.length;i++){
//。。。。
}
}
ES6 new features introduced, in addition to obtaining all parameters - parameters that have been defined ....
function aaa(a,b,...rest) {
console.log("a=>"+a);
console.log("b=>"+b);
console.log(rest);
}
rest parameters can only write in the final surface, it must be used ... to identify.
2. The scope of variables
In javascript, var actual variable is defined scope.
Suppose statement in the function body, the function can not be used in vitro - (To achieve non then can look back 闭包)
function yy() {
var x = 1;
x = x + 1;
}
x = x + 2; //Uncaught ReferenceError: x is not defined
If the two functions use the same variable name, as long as the internal function, not conflict
function yy() {
var x = 1;
x = x + 1;
}
function yy1() {
var x = 'A';
x = x + 1;
}
Internal functions can access external member function, not vice versa
function yy() {
var x = 1;
// 内部函数可以访问外部函数的成员，反之则不行
function yy2() {
var y = x + 1; // 2
}
var z = y + 1; // Uncaught ReferenceError: y is not defined
}
Assume that the variables inside the function variables and external functions of the same name!
function yy() {
var x = 1;
function yy2() {
var x = 'A';
console.log('inner'+x); // outer1
}
console.log('outer'+x); //innerA
yy2()
}
yy()
Suppose in JavaScript functions for variables from their own functions - start, '' the 'variable with' to 'outside' look. Suppose that the external variable function of the same name exists, the internal function of shielding external function.
To enhance the scope of variables
function yy() {
var x = "x" + y;
console.log(x);
var y = 'y';
}
The results: xundefined
Description; js execution engine automatically enhance the declaration of y, but will not improve the assignment variable y ';
function qj2() {
var y;
var x = "x" + y;
console.log(x);
y = 'y';
}
This is the beginning of JavaScript build on existing properties. Develop specifications: all variables are defined at the head of the function, not misplacing easy code maintenance;
function yy2() {
var x = 1,
y = x + 1,
z,i,a; //undefined
// 之后随意用
}
Global Functions
//全局变量
x = 1;
function f() {
console.log(x);
}
f();
console.log(x);
Global object window
var x = 'xxx';
alert(x);
alert(window.x); // 默认所有的全局变量，都会自动绑定在 window对象下；
alert () function itself is a windowvariable;
var x = 'xxx';
window.alert(x);
var old_alert = window.alert;
//old_alert(x);
window.alert = function () {
};
// 发现 alert() 失效了
window.alert(123);
//恢复
window.alert = old_alert;
window.alert(456);
Javascript is actually only a global scope, any variable (function can also be considered variable), assuming that is not found in the function scope, will find out, if not found in global scope, error RefrenceError
specification
Since all of our global variables are bound to our window. If different js files, use the same global variables, conflict -> If you can reduce conflict?
// 唯一全局变量
var KuangApp = {};
// 定义全局变量
KuangApp.name = 'yy';
KuangApp.add = function (a,b) {
return a + b;
}
Put all your code into their own unique name space definition, reducing the global problem of naming conflicts -
jQuery
Let local scope
function aaa() {
for (var i = 0; i < 100; i++) {
console.log(i)
}
console.log(i+1); //问题？ i 出了这个作用域还可以使用
}
ES6 let keyword, local scope resolve conflicts!
function aaa() {
for (let i = 0; i < 100; i++) {
console.log(i)
}
console.log(i+1); //Uncaught ReferenceError: i is not defined
}
I suggest that you are used letto define a local variable scope;
Constant const
Before ES6, how to define constants: Only use all capital letters named variable is constant; recommended not to modify this value
var PI = '3.14';
console.log(PI);
PI = '213'; //可以改变这个值
console.log(PI);
In ES6 introduced the constant keyword const
const PI = '3.14'; // 只读变量
console.log(PI);
PI = '123'; // TypeError: Assignment to constant variable.
console.log(PI);
3. Methods
Definition method
The method is to function in an object inside the object are only two things: the properties and methods
var yy = {
name: '洋',
bitrh: 2000,
// 方法
age: function () {
// 今年 - 出生的年
var now = new Date().getFullYear();
return now-this.bitrh;
}
}
//属性
yy.name
//方法,一定要带 ()
yy.age()
this. What representatives? Apart above code see ~
function getAge() {
// 今年 - 出生的年
var now = new Date().getFullYear();
return now-this.bitrh;
}
var yy = {
name: '洋',
bitrh: 2000,
age: getAge
}
// yy.age() ok
// getAge() NaN window
this is not the point, is that it points to the default object calls;
apply
In js can control this point!
function getAge() {
// 今年 - 出生的年
var now = new Date().getFullYear();
return now-this.bitrh;
}
var yy = {
name: '洋',
bitrh: 2000,
age: getAge
};
var xiaoming = {
name: '小明',
bitrh: 2000,
age: getAge
};
// yy.age() ok
getAge.apply(xiaoming,[]);// this，指向了 yy，参数为空
