Symbol用来标识一个唯一标识例如id
let id = Symbol();
Symbol也可以按名字命名
// id is a symbol with the description "id" let id = Symbol("id");
即时描述一样，也表示为不同的值
let id1 = Symbol("id"); let id2 = Symbol("id"); alert(id1 == id2); // false
Symbol不能自动转换为字符串
let id = Symbol("id"); alert(id); // TypeError: Cannot convert a Symbol value to a string
但是可以显示的调用方法获取值
let id = Symbol("id"); alert(id.toString()); // Symbol(id), now it works alert(id.description); // id
Symbol可以作为对象的影藏属性
let user = { // belongs to another code name: "John" }; let id = Symbol("id"); user[id] = 1; alert( user[id] ); // we can access the data using the symbol as the key
如果在对象中定义需要使用中括号
let id = Symbol("id"); let user = { name: "John", [id]: 123 // not "id": 123 };
在for…in循环中不会读取Symbol
let id = Symbol("id"); let user = { name: "John", age: 30, [id]: 123 }; for (let key in user) alert(key); // name, age (no symbols) // the direct access by the symbol works alert( "Direct: " + user[id] );
对象拷贝的时候Symbol也会被拷贝
let id = Symbol("id"); let user = { [id]: 123 }; let clone = Object.assign({}, user); alert( clone[id] ); // 123
全局Symbol定义的值是一样的
// read from the global registry let id = Symbol.for("id"); // if the symbol did not exist, it is created // read it again (maybe from another part of the code) let idAgain = Symbol.for("id"); // the same symbol alert( id === idAgain ); // true
访问全局Symbol
// get symbol by name let sym = Symbol.for("name"); let sym2 = Symbol.for("id"); // get name by symbol alert( Symbol.keyFor(sym) ); // name alert( Symbol.keyFor(sym2) ); // id
let globalSymbol = Symbol.for("name"); let localSymbol = Symbol("name"); alert( Symbol.keyFor(globalSymbol) ); // name, global symbol alert( Symbol.keyFor(localSymbol) ); // undefined, not global alert( localSymbol.description ); // name
keyFor只能访问到全局的，访问不到局部的
