是什么导致此 f.apply 中的“非法调用错误”

Question

'use strict';

function delay(f, ms) {

  return function() {
    setTimeout(() => f.apply(this, arguments), ms); //Illegal invocation
  };

}

let obj = {length: 20};
obj.f1000 = delay(alert, 1000);

obj.f1000(this.length); 

Why do I get illegal invocation error? If the value of this is obj, then the function should just alert "20" without any problems?

Answer 1

Because you're calling alert with a this value it doesn't expect: your object. The reason that's happening is that you're calling the traditional function that delay returns as a method ( obj.f1000 ), which means during the call to the function this refers to the object, and you're passing this as the first argument to apply in f.apply(this, arguments) . So alert sees your object as this .

To fix it, use null or undefined instead of this with apply :

 'use strict'; function delay(f, ms) { return function() { setTimeout(() => f.apply(null, arguments), ms); //Illegal invocation // −−−−−−−−−−−−−−−−−−−−−−−−−−^^^^ }; } let obj = {length: 20}; obj.f1000 = delay(alert, 1000); obj.f1000(this.length);

(You could also use window if you wanted.)