Why invoke apply instead of calling function directly?

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I think the explanation from the  Mozilla Docs  describes it well:

You can assign a different  this  object when calling an existing function.  this  refers to the current object, the calling object. With apply, you can write a method once and then inherit it in another object, without having to rewrite the method for the new object.
apply is very similar to call, except for the type of arguments it supports. You can use an arguments array instead of a named set of parameters. With apply, you can use an array literal, for example, fun.apply(this, [name, value]), or an Array object, for example, fun.apply(this, new Array(name, value)).
As for the parameters:
thisArg  Determines the value of  this  inside fun. If  thisArg  is null or undefined,  this  will be the global object. Otherwise, this will be equal to Object(thisArg) (which is thisArg if thisArg is already an object, or a String, Boolean, or Number if thisArg is a primitive value of the corresponding type). Therefore, it is always true that typeof this == "object" when the function executes.
argsArray  An argument array for the object, specifying the arguments with which fun should be called, or null or undefined if no arguments should be provided to the function.
The docs give a good example of a use case for apply. In the example below, apply is used to chain a constructor:
function product(name, value) { this.name = name; if (value > = 1000) this.value = https://www.songbingjia.com/android/999; else this.value = value; }function prod_dept(name, value, dept) { this.dept = dept; product.apply(this, arguments); } prod_dept.prototype = new product(); // since 5 is less than 1000 value is set var cheese = new prod_dept("feta", 5, "food"); // since 5000 is above 1000, value will be 999 var car = new prod_dept("honda", 5000, "auto");

【Why invoke apply instead of calling function directly?】Notice that in the prod_dept constructor, the this supplied refers to the prod_dept object, and arguments is an array of arguments passed to the product constructor.

 
 
 
现象
经常在网上或者阅读源码时看到下面的代码:
Array.prototype.slice.call(arr, 3);

而不是
arr.slice(3);

原因
这是为什么呢, 毕竟下面这种方法更短, 代码看起来也更清晰.
因为使用prototype.fn.call这种方式可以更好的复用代码, 下面这个例子:
var divs = document.getElementsByTagName(‘div‘); var first_three_div = divs.slice(3) // TypeError: divs.slice is not a function var first_three_div = Array.prototype.slice.call(divs, 3); // ok

这里  divs  是一个  htmlCollection  对象, 但不是一个  Array  对象, 因此直接调用  slice  会出现错误, 而使用  Array.prototype.slice.call  我们可以复用  Array  的方法实现  HTMLCollection  对象的  slice方法. 这是因为我们使用  call  调用原型方法, 使  this  指针指向  HTMLCollection  对象, 而且这个对象正好拥有  length  属性和通过数字下标获取元素的方法, 因此  slice  能够返回正确的结果.
总结
通过原型方法调用可以复用其他类型的方法, 比如非  Array  类型可以使用  slice  方法获得  slice  的功能.
参考
javascript-why-use-prototype-to-call-a-function-instead-of-just-calling-the-fun

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