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Editorial Team
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Asked: 2026-06-06T12:59:17+00:00

This question has disturbed me for a long time. Sorry if it is a

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This question has disturbed me for a long time. Sorry if it is a stupid question.

Before, I have known that you can get the elements with a class name

document.body.getElementsByClassName("foo");

And I was so lazy, so I just copied and pasted the code to the other part to do this

document.body.getElementById("bar");

I accidentally found it won’t work. I tested and it says

TypeError: Object #<HTMLBodyElement> has no method ‘getElementById’

So why does it have getElementsByClassName andgetElementsByTagName and all those similar methods, but only no getElementById?

typeof document === typeof document.body   //true

Their types are the same, so they should have the same thing. But it does not seem to be the case here.

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1 Answer

  1. Editorial Team

    You can have multiple elements with the same class name so narrowing down the search to start with a specific node make sense.

    It doesn’t make sense with id because it it should be unique.

    You can have only one id in the document, this why getElementById is a method of document.

    Example:

    <body>
    <div id="start">
        <span class="a">
    </div>
    <div class="a">
    </div>
</body>

    Start searching for class a from the node <div id="start"> will give you one element,
    While if you would have start from the top node- document, it would have ended with two elements.

    Regarding to the typeof comparing:

    typeof 1 == typeof 2 == "Number" // true
1 !== 2 // true.

    typeof only checks for the type, not the value, document and document.body are both objects, but different objects.

    typeof document === typeof document.body === typeof null === "object" // true
document === document.body // false!!!

    As you can see, null and document share the same type, but do they have the same methods…? NO

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