为什么我不能在箭头函数中省略getElementById（）的参数？ [英] Why can't I omit the parameter of getElementById() in the Arrow Function?

问题描述

假设我在页面上有3个< img> 标记，我想将这些标记作为数组，所以我写道：

Say I have 3 <img> tags on a page, and I would like to get these as an array, so I wrote:

让myArray = ['img1'，'img2'，'img3']。map（id => document.getElementById（id））;

...运作良好。

然后我想，嘿， getElementById 只需1个参数。是不是有语法糖？所以我写道：

Then I thought, hey, getElementById takes exactly 1 argument. Isn't there a syntax sugar for that? So I wrote:

让myArray = ['img1'，'img2'，'img3']。map（document.getElementById）;

......但这不起作用。我在Chrome上收到了非法调用。

... but that didn't work. I got "Illegal invocation" on Chrome.

所以它不是语法糖。所有这些背后的原因是什么？

So it's not syntax sugar then. What's behind all these?

推荐答案

JavaScript在方法调用和函数调用之间存在差异。前者将设置为，后者则不会。从语法上讲，方法调用必须是 receiver.method（... args）。没有点，没有方法调用。所以，这个：

JavaScript has a difference between "method call" and "function call". The former will set this, the latter won't. Syntactically, method call must be of form receiver.method(...args). No dot, no method call. So, this:

document.getElementById(id) // method call, `this` is set to `document`
m = document.getElementById; m(id) // function call, `this` is not set

当你执行<$ c时$ c> map（document.getElementById）， document.getElementById 是从其对象中提取的函数;当 map 调用它时，它将在没有接收器的情况下调用它，这个将不会被设置，并且事情变得糟糕。

When you do map(document.getElementById), document.getElementById is a function plucked from its object; when map invokes it, it will invoke it without the receiver, this will not be set, and things get bad.

有一种方法可以保存它： bind ，它通过将接收器绑定到它来方法化一个函数： map（document.getElementById.bind（document））应该可以工作。

There is a way to save it: bind, which "methodifies" a function by binding a receiver to it: map(document.getElementById.bind(document)) should work.

编辑：进一步说明：

let foo = {
  bar: function(context) {
    let receiver =
      (this == window) ? "window" :
      (this == foo) ? "foo" :
      "unknown";
    console.log(context + ", `this` is `" + receiver + "`");
  }
}

function call(fn, param) {
  fn(param);
}

foo.bar("In direct call");
let baz = foo.bar; baz("When assigned to a variable");
call(foo.bar, "When passed as a parameter")
let quux = foo.bar.bind(foo); quux("When bound to foo");

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