Strict Mode

Strict Mode

作用

Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. 这种严格的上下文能够禁掉一些行为以及抛出更多地错误.

为什么需要它

Strict mode makes it easier to write “secure” JavaScript.

如何开启

文件顶部：

// File: myscript.js

'use strict';
var a = 2;
...

function 顶部：

function doSomething() {
    'use strict';
    ...
}

约束

禁止全局变量

'use strict';

// Assignment to a non-writable global
var undefined = 5; // throws a TypeError
var Infinity = 5; // throws a TypeError

delete 不可删除的属性，会抛出异常。例如尝试 delete Object.prototype

'use strict';
delete Object.prototype; // throws a TypeError

object literal 中的所有属性名称应该是唯一的。反例 var x = { "x1": 1, "x1": 2 }

'use strict';
var o = { p: 1, p: 2 }; // !!! syntax error

函数参数的名字必须唯一。反例 function sum(x, x) {}

function sum(a, a, c) { // !!! syntax error
  'use strict';
  return a + a + c; // wrong if this code ran
}

禁用八进制声明 var x = 023

'use strict';
var sum = 015 + // !!! syntax error
          197 +
          142;

禁用 with

'use strict';
var x = 17;
with (obj) { // !!! syntax error
  // If this weren't strict mode, would this be var x, or
  // would it instead be obj.x?  It's impossible in general
  // to say without running the code, so the name can't be
  // optimized.
  x;
}

eval 不会产生新的变量
禁止 delete 普通变量。反例 delete x

'use strict';

var x;
delete x; // !!! syntax error

Forbids binding or assignment of the names eval and arguments in any form
Strict mode does not alias properties of the arguments object with the formal parameters. (i.e. in function sum (a,b) { return arguments[0] + b;} This works because arguments[0] is bound to a and so on. )
不支持 arguments.callee

'use strict';
function fun(a, b) {
  'use strict';
  var v = 12;
  return arguments.caller; // throws a TypeError
}
fun(1, 2); // doesn't expose v (or a or b)

function restricted() {
  'use strict';
  restricted.caller;    // throws a TypeError
  restricted.arguments; // throws a TypeError
}

参考