【ECMAScript6标准入门】2.const的使用

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//2.3.1const用于声明一个常量，不量声明，常量的值不能改变。const PI = 3.1415926;//声明的同时就必须给予赋值//PI = 0;//报错，TypeError: Assignment to constant variable.console.log(PI);//作用域与let相同{const a = "123";}//console.log(a);//报错,ReferenceError: a is not definedvar message = "Hello";let age = 15;//以下两行都会报错 //const message = "world";//报错,SyntaxError: Identifier 'message' has already been declared//const age = 10;//报错,Duplicate declaration "age//2.3.2本质//const实际上保证的并不是变量的值不得改动，而是变量指向的那个内存地址不得改动。const foo = {}foo.prop = 123;foo.prop = 432;console.log(foo);//{ prop: 432 }const arr = [];arr.push("hello");//可执行arr.length = 0;//可执行//arr = ["adf"];//报错，TypeError: Assignment to constant variable.console.log(arr);const foo2 = Object.freeze({})//将对象冻结foo2.prop = 123;console.log(foo2);//{}const arr2 = Object.freeze([]);//arr2.push("hello");//报错，TypeError: Can't add property 0, object is not extensibleconsole.log(arr2);//[]//将对象彻底冻结函数var constantize = (obj) => {Object.freeze(obj);Object.keys(obj).forEach((key,i) => {if (typeof obj[key] === 'object') {constantize(obj[key]);}});};

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整理内容摘自《ES6标准入门》第三版阮一峰

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