javascript - 如何在调用函数中设置空上下文？

Question

function test(){         
     if(this === null){
        console.log("This is null");
     }else{
         console.log("This is Object");
     }
 }
test.call(null);
test.call({});

输出:

This is Object.

This is Object.

但我希望输出:

This is Null.

This is Object.

为什么不在上下文中设置 Null？

Answer 1

引自MDN

if the method is a function in non-strict mode, null and undefined will be replaced with the global object and primitive values will be converted to objects.

这解释了为什么在调用 test.call(null); 时会得到一个对象。当此处传递null时，test()中的this将是全局对象Window。

对于所需的行为，请使用严格模式。

function test() {
  "use strict";
  if (this === null) {
    console.log("This is null");
  } else {
    console.log("This is Object");
  }
}
test.call(null);
test.call({});

引用自 ES6 规范 strict mode

If this is evaluated within strict mode code, then the this value is not coerced to an object. A this value of null or undefined is not converted to the global object and primitive values are not converted to wrapper objects. The this value passed via a function call (including calls made using Function.prototype.apply and Function.prototype.call) do not coerce the passed this value to an object