javascript - What is the difference between querySelectorAll and getElementsByTagName?

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javascript - What is the difference between querySelectorAll and getElementsByTagName?

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

javascript - What is the difference between querySelectorAll and getElementsByTagName?

I was wondering about two different syntax of selecting element in JavaScript.

suppose if I want to select all divs from current document then:

var divs = document.getElementsByTagName("div");
console.log("There are "+divs.length+" Divs in Document !");

Will work fine. But there is also another way of doing so, like:

var divs = document.querySelectorAll("div");
console.log("There are "+divs.length+" Divs in Document !");

When both of them works in the same way. What's the difference between them ?

Which one is faster? Why? How both works?

Thanks in advance. I've seen the questions like this but they didn't satisfied the need.

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1 Answer

深蓝 · Answer 1 · 2021-10-23T18:30:36+0000

Most answeres are wrong. Nicolae Olariu is the only, who answered correcly

Which one is faster? Why?

are not the questions. The real question "How it works?"

The main difference is in this example:

<!doctype html> 
<html> 
<head> 
    <meta charset="utf-8"> 
    <title>Yandex</title> 

</head> 
<body> 
    <a href="((http://yandex.ru))">Яндекс</a>, 
    <a href="((http://yandex.com))">Yandex</a> 
</body> 
<script>

var elems1 = document.getElementsByTagName('a'), // return 2 lements, elems1.length = 2 
    elems2 = document.querySelectorAll("a");  // return 2 elements, elems2.length = 2 

document.body.appendChild(document.createElement("a")); 

console.log(elems1.length, elems2.length);  // now elems1.length = 3! 
                                            // while elems2.length = 2
</script>
</html>

Because querySelectorAll returns a static (not live) list of elements.

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javascript - What is the difference between querySelectorAll and getElementsByTagName?

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