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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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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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