Set document.getElementById to variable

问题

The following works:

    $ = document.form;
    x = $.name.value;

This doesn't:

    $ = document.getElementById;
    x = $("id").value;

Any ideas on why this doesn't work or how to make it so?

回答1:

The value of this depends on how you call the function.

When you call document.getElementById then getElementById gets this === document. When you copy getElementById to a different variable and then call it as $ then this === window (because window is the default variable).

This then causes it to look for the id in the window object instead of in the document object, and that fails horribly because windows aren't documents and don't have the same methods.

You need to maintain the document in the call. You can use a wrapper functions for this e.g.

function $ (id) { return document.getElementById(id); }

… but please don't use $. It is a horrible name. It has no meaning and it will confuse people who see it and think "Ah! I know jQuery!" or "Ah! I know Prototype" or etc etc.

回答2:

The context object is different. When you get a reference of a function you're changing that context object:

var john = {
    name : "john",
    hello : function () { return "hello, I'm " + this.name }
}

var peter = { name : "peter" };

peter.hello = john.hello;

peter.hello() // "hello, I'm peter"

If you want a reference function bound to a specific context object, you have to use bind:

peter.hello = john.hello.bind(john);

peter.hello(); // "hello, I'm john"

So in your case it will be:

var $ = document.getElementById.bind(document);

回答3:

getElementById is a method of the HTMLDocument prototype (of which document is an instance). So, calling the function in global context you will surely get an "Wrong this Error" or something.

You may use

var $ = document.getElementById.bind(document);

but

function $(id) { return document.getElementById(id); }

is also OK and maybe better to understand.

回答4:

Don't know what you want to achieve, but this can be made working like this

$ = document.getElementById;
x = $.call(document, "id").value;

because getElementById works only when it is a function of document because of the scope it needs.

But I would recommend @Quentin's answer.

回答5:

If you are trying to achieve something like that I would suggest using jQuery. Their $ notation is much more powerful than just getting an element by id.

Also, if you are using any platform that already uses the $ as a variable (ASP .Net sometimes uses this) you may have unpredictable result.

来源：https://stackoverflow.com/questions/9804828/set-document-getelementbyid-to-variable

标签

javascript

variables

getelementbyid