ECMAScript 6 入门教程—对象的扩展
作者 | 阮一峰
Object对象的新增方法。1、属性的简洁表示法
const foo = 'bar';
const baz = {foo};
baz // {foo: "bar"}
// 等同于
const baz = {foo: foo};
function f(x, y) {
return {x, y};
}
// 等同于
function f(x, y) {
return {x: x, y: y};
}
f(1, 2) // Object {x: 1, y: 2}
const o = {
method() {
return "Hello!";
}
};
// 等同于
const o = {
method: function() {
return "Hello!";
}
};
let birth = '2000/01/01';
const Person = {
name: '张三',
//等同于birth: birth
birth,
// 等同于hello: function ()...
hello() { console.log('我的名字是', this.name); }
};
function getPoint() {
const x = 1;
const y = 10;
return {x, y};
}
getPoint()
// {x:1, y:10}
let ms = {};
function getItem (key) {
return key in ms ? ms[key] : null;
}
function setItem (key, value) {
ms[key] = value;
}
function clear () {
ms = {};
}
module.exports = { getItem, setItem, clear };
// 等同于
module.exports = {
getItem: getItem,
setItem: setItem,
clear: clear
};
const cart = {
_wheels: 4,
get wheels () {
return this._wheels;
},
set wheels (value) {
if (value < this._wheels) {
throw new Error('数值太小了！');
}
this._wheels = value;
}
}
let user = {
name: 'test'
};
let foo = {
bar: 'baz'
};
console.log(user, foo)
// {name: "test"} {bar: "baz"}
console.log({user, foo})
// {user: {name: "test"}, foo: {bar: "baz"}}
const obj = {
f() {
this.foo = 'bar';
}
};
new obj.f() // 报错
2、属性名表达式
// 方法一
obj.foo = true;
// 方法二
obj['a' + 'bc'] = 123;
var obj = {
foo: true,
abc: 123
};
let propKey = 'foo';
let obj = {
[propKey]: true,
['a' + 'bc']: 123
};
let lastWord = 'last word';
const a = {
'first word': 'hello',
[lastWord]: 'world'
};
a['first word'] // "hello"
a[lastWord] // "world"
a['last word'] // "world"
let obj = {
['h' + 'ello']() {
return 'hi';
}
};
obj.hello() // hi
// 报错
const foo = 'bar';
const bar = 'abc';
const baz = { [foo] };
// 正确
const foo = 'bar';
const baz = { [foo]: 'abc'};
const keyA = {a: 1};
const keyB = {b: 2};
const myObject = {
[keyA]: 'valueA',
[keyB]: 'valueB'
};
myObject // Object {[object Object]: "valueB"}
3、方法的 name 属性
const person = {
sayName() {
console.log('hello!');
},
};
person.sayName.name // "sayName"
const obj = {
get foo() {},
set foo(x) {}
};
obj.foo.name
// TypeError: Cannot read property 'name' of undefined
const descriptor = Object.getOwnPropertyDescriptor(obj, 'foo');
descriptor.get.name // "get foo"
descriptor.set.name // "set foo"
(new Function()).name // "anonymous"
var doSomething = function() {
// ...
};
doSomething.bind().name // "bound doSomething"
const key1 = Symbol('description');
const key2 = Symbol();
let obj = {
[key1]() {},
[key2]() {},
};
obj[key1].name // "[description]"
obj[key2].name // ""
4、属性的可枚举性和遍历
可枚举性
let obj = { foo: 123 };
Object.getOwnPropertyDescriptor(obj, 'foo')
// {
// value: 123,
// writable: true,
// enumerable: true,
// configurable: true
// }
for...in循环：只遍历对象自身的和继承的可枚举的属性。
Object.keys()：返回对象自身的所有可枚举的属性的键名。
JSON.stringify()：只串行化对象自身的可枚举的属性。
Object.assign()： 忽略enumerable为false的属性，只拷贝对象自身的可枚举的属性。
Object.getOwnPropertyDescriptor(Object.prototype, 'toString').enumerable
// false
Object.getOwnPropertyDescriptor([], 'length').enumerable
// false
Object.getOwnPropertyDescriptor(class {foo() {}}.prototype, 'foo').enumerable
// false
属性的遍历
首先遍历所有数值键，按照数值升序排列。
其次遍历所有字符串键，按照加入时间升序排列。
最后遍历所有 Symbol 键，按照加入时间升序排列。
Reflect.ownKeys({ [Symbol()]:0, b:0, 10:0, 2:0, a:0 })
// ['2', '10', 'b', 'a', Symbol()]
5、super 关键字
const proto = {
foo: 'hello'
};
const obj = {
foo: 'world',
find() {
return super.foo;
}
};
Object.setPrototypeOf(obj, proto);
obj.find() // "hello"
// 报错
const obj = {
foo: super.foo
}
// 报错
const obj = {
foo: () => super.foo
}
// 报错
const obj = {
foo: function () {
return super.foo
}
}
const proto = {
x: 'hello',
foo() {
console.log(this.x);
},
};
const obj = {
x: 'world',
foo() {
super.foo();
}
}
Object.setPrototypeOf(obj, proto);
obj.foo() // "world"
6、对象的扩展运算符
解构赋值
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
x // 1
y // 2
z // { a: 3, b: 4 }
let { ...z } = null; // 运行时错误
let { ...z } = undefined; // 运行时错误
let { ...x, y, z } = someObject; // 句法错误
let { x, ...y, ...z } = someObject; // 句法错误
let obj = { a: { b: 1 } };
let { ...x } = obj;
obj.a.b = 2;
x.a.b // 2
let o1 = { a: 1 };
let o2 = { b: 2 };
o2.__proto__ = o1;
let { ...o3 } = o2;
o3 // { b: 2 }
o3.a // undefined
const o = Object.create({ x: 1, y: 2 });
o.z = 3;
let { x, ...newObj } = o;
let { y, z } = newObj;
x // 1
y // undefined
z // 3
let { x, ...{ y, z } } = o;
// SyntaxError: ... must be followed by an identifier in declaration contexts
function baseFunction({ a, b }) {
// ...
}
function wrapperFunction({ x, y, ...restConfig }) {
// 使用 x 和 y 参数进行操作
// 其余参数传给原始函数
return baseFunction(restConfig);
}
扩展运算符
let z = { a: 3, b: 4 };
let n = { ...z };
n // { a: 3, b: 4 }
let foo = { ...['a', 'b', 'c'] };
foo
// {0: "a", 1: "b", 2: "c"}
{...{}, a: 1}
// { a: 1 }
// 等同于 {...Object(1)}
{...1} // {}
// 等同于 {...Object(true)}
{...true} // {}
// 等同于 {...Object(undefined)}
{...undefined} // {}
// 等同于 {...Object(null)}
{...null} // {}
{...'hello'}
// {0: "h", 1: "e", 2: "l", 3: "l", 4: "o"}
let aClone = { ...a };
// 等同于
let aClone = Object.assign({}, a);
// 写法一
const clone1 = {
__proto__: Object.getPrototypeOf(obj),
...obj
};
// 写法二
const clone2 = Object.assign(
Object.create(Object.getPrototypeOf(obj)),
obj
);
// 写法三
const clone3 = Object.create(
Object.getPrototypeOf(obj),
Object.getOwnPropertyDescriptors(obj)
)
let ab = { ...a, ...b };
// 等同于
let ab = Object.assign({}, a, b);
let aWithOverrides = { ...a, x: 1, y: 2 };
// 等同于
let aWithOverrides = { ...a, ...{ x: 1, y: 2 } };
// 等同于
let x = 1, y = 2, aWithOverrides = { ...a, x, y };
// 等同于
let aWithOverrides = Object.assign({}, a, { x: 1, y: 2 });
let newVersion = {
...previousVersion,
name: 'New Name' // Override the name property
};
let aWithDefaults = { x: 1, y: 2, ...a };
// 等同于
let aWithDefaults = Object.assign({}, { x: 1, y: 2 }, a);
// 等同于
let aWithDefaults = Object.assign({ x: 1, y: 2 }, a);
const obj = {
...(x > 1 ? {a: 1} : {}),
b: 2,
};
let a = {
get x() {
throw new Error('not throw yet');
}
}
let aWithXGetter = { ...a }; // 报错
7、链判断运算符
// 错误的写法
const firstName = message.body.user.firstName;
// 正确的写法
const firstName = (message
&& message.body
&& message.body.user
&& message.body.user.firstName) || 'default';
const fooInput = myForm.querySelector('input[name=foo]')
const fooValue = fooInput ? fooInput.value : undefined
const firstName = message?.body?.user?.firstName || 'default';
const fooValue = myForm.querySelector('input[name=foo]')?.value
iterator.return?.()
if (myForm.checkValidity?.() === false) {
// 表单校验失败
return;
}
obj?.prop // 对象属性
obj?.[expr] // 同上
func?.(...args) // 函数或对象方法的调用
let hex = "#C0FFEE".match(/#([A-Z]+)/i)?.[1];
a?.b
// 等同于
a == null ? undefined : a.b
a?.[x]
// 等同于
a == null ? undefined : a[x]
a?.b()
// 等同于
a == null ? undefined : a.b()
a?.()
// 等同于
a == null ? undefined : a()
a?.[++x]
// 等同于
a == null ? undefined : a[++x]
delete a?.b
// 等同于
a == null ? undefined : delete a.b
(a?.b).c
// 等价于
(a == null ? undefined : a.b).c
// 构造函数
new a?.()
new a?.b()
// 链判断运算符的右侧有模板字符串
a?.`{b}`
a?.b`{c}`
// 链判断运算符的左侧是 super
super?.()
super?.foo
// 链运算符用于赋值运算符左侧
a?.b = c
8、Null 判断运算符
const headerText = response.settings.headerText || 'Hello, world!';
const animationDuration = response.settings.animationDuration || 300;
const showSplashScreen = response.settings.showSplashScreen || true;
const headerText = response.settings.headerText ?? 'Hello, world!';
const animationDuration = response.settings.animationDuration ?? 300;
const showSplashScreen = response.settings.showSplashScreen ?? true;
const animationDuration = response.settings?.animationDuration ?? 300;
function Component(props) {
const enable = props.enabled ?? true;
// …
}
function Component(props) {
const {
enabled: enable = true,
} = props;
// …
}
// 报错
lhs && middle ?? rhs
lhs ?? middle && rhs
lhs || middle ?? rhs
lhs ?? middle || rhs
(lhs && middle) ?? rhs;
lhs && (middle ?? rhs);
(lhs ?? middle) && rhs;
lhs ?? (middle && rhs);
(lhs || middle) ?? rhs;
lhs || (middle ?? rhs);
(lhs ?? middle) || rhs;
lhs ?? (middle || rhs);
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