常量
// ES5 中常量的写法 Object.defineProperty(window, "PI2", { value: 3.1415926, writable: false, }) console.log(window.PI2) // ES6 的常量写法 const PI = 3.1415926 console.log(PI) // PI = 4
作用域
// ES5 中作用域
const callbacks = []for (var i = 0; i <= 2; i++) {
callbacks[i] = function() {
return i * 2
}
}
console.table([
callbacks[0](),
callbacks[1](),
callbacks[2](),
])
// 输出是6 6 6
const callbacks2 = []
for (let j = 0; j <= 2; j++) {
callbacks2[j] = function() {
return j * 2
}
}
console.table([
callbacks2[0](),
callbacks2[1](),
callbacks2[2](),
])
// 输出是0 2 4
;((function() {
const foo = function() {
return 1
}
console.log("foo()===1", foo() === 1)
;((function() {
const foo = function() {
return 2
}
console.log("foo()===2", foo() === 2)
})())
})())
{
function foo() {
return 1
}
console.log("foo()===1", foo() === 1)
{
function foo() {
return 2
}
console.log("foo()===2", foo() === 2)
}
console.log("foo()===1", foo() === 1)
}
箭头函数
/ eslint-disable /
{
// ES3,ES5
var evens = [1, 2, 3, 4, 5];
var odds = evens.map(function(v) {
return v + 1
});
console.log(evens, odds);
};
{
// ES6
let evens = [1, 2, 3, 4, 5];
let odds = evens.map(v => v + 1);
console.log(evens, odds);
}
{
// ES3,ES5
var factory = function() {
this.a = 'a';
this.b = 'b';
this.c = {
a: 'a+',
b: function() {
return this.a
}
}
}
// 谁调用this指向谁
console.log(new factory().c.b()); // 'a+'
};
{
// ES6
var factory = function() {
this.a = 'a';
this.b = 'b';
this.c = {
a: 'a+',
b: () => {
return this.a
}
}
}
// 定义时指向谁就是谁
console.log(new factory().c.b()); // 'a'
}
函数参数
/ eslint-disable /
{
// ES5\ES3 默认参数的写法
function f(x, y, z) {
if (y === undefined) {
y = 7;
}
if (z === undefined) {
z = 42
}
return x + y + z
}
console.log(f(1, 3));
}
{
// ES6 默认参数
function f(x, y = 7, z = 42) {
return x + y + z
}
console.log(f(1, 3));
}
{
// 必选参数检查
function checkParameter() {
throw new Error('can\'t be empty')
}
function f(x = checkParameter(), y = 7, z = 42) {
return x + y + z
}
console.log(f(1));
try {
f()
} catch (e) {
console.log(e);
} finally {}
}
{
// ES3,ES5 可变参数
function f() {
var a = Array.prototype.slice.call(arguments);
var sum = 0;
a.forEach(function(item) {
sum += item * 1;
})
return sum
}
console.log(f(1, 2, 3, 6));
}
{
// ES6 可变参数
function f(...a) {
var sum = 0;
a.forEach(item => {
sum += item * 1
});
return sum
}
console.log(f(1, 2, 3, 6));
}
{
// ES5 合并数组
var params = ['hello', true, 7];
var other = [1, 2].concat(params);
console.log(other);
}
{
// ES6 利用扩展运算符合并数组
var params = ['hello', true, 7];
var other = [
1, 2, ...params
];
console.log(other);
}
数据保护
/ eslint-disable /
{
// ES3,ES5 数据保护
var Person = function() {
var data = {
name: 'es3',
sex: 'male',
age: 15
}
this.get = function(key) {
return data[key]
}
this.set = function(key, value) {
if (key !== 'sex') {
data[key] = value
}
}
}
// 声明一个实例
var person = new Person();
// 读取
console.table({name: person.get('name'), sex: person.get('sex'), age: person.get('age')});
// 修改
person.set('name', 'es3-cname');
console.table({name: person.get('name'), sex: person.get('sex'), age: person.get('age')});
person.set('sex', 'female');
console.table({name: person.get('name'), sex: person.get('sex'), age: person.get('age')});
} {
// ES5
var Person = {
name: 'es5',
age: 15
};
Object.defineProperty(Person, 'sex', {
writable: false,
value: 'male'
});
console.table({name: Person.name, age: Person.age, sex: Person.sex});
Person.name = 'es5-cname';
console.table({name: Person.name, age: Person.age, sex: Person.sex});
try {
Person.sex = 'female';
console.table({name: Person.name, age: Person.age, sex: Person.sex});
} catch (e) {
console.log(e);
}
} {
// ES6
let Person = {
name: 'es6',
sex: 'male',
age: 15
};
let person = new Proxy(Person, {
get(target, key) {
return target[key]
},
set(target,key,value){
if(key!=='sex'){
target[key]=value;
}
}
});
console.table({
name:person.name,
sex:person.sex,
age:person.age
});
try {
person.sex='female';
} catch (e) {
console.log(e);
} finally {
}
}
</div>
