How do you test an element for existence without the use of the getElementById method?
(如何在不使用getElementById方法的情况下测试元素是否存在？)
(我已经设置了一个现场演示供参考。)
I will also print the code on here as well:(我也会在这里打印代码：)
<!DOCTYPE html>
<html>
<head>
<script>
var getRandomID = function (size) {
var str = "",
i = 0,
chars = "0123456789abcdefghijklmnopqurstuvwxyzABCDEFGHIJKLMNOPQURSTUVWXYZ";
while (i < size) {
str += chars.substr(Math.floor(Math.random() * 62), 1);
i++;
}
return str;
},
isNull = function (element) {
var randomID = getRandomID(12),
savedID = (element.id)? element.id : null;
element.id = randomID;
var foundElm = document.getElementById(randomID);
element.removeAttribute('id');
if (savedID !== null) {
element.id = savedID;
}
return (foundElm) ? false : true;
};
window.onload = function () {
var image = document.getElementById("demo");
console.log('undefined', (typeof image === 'undefined') ? true : false); // false
console.log('null', (image === null) ? true : false); // false
console.log('find-by-id', isNull(image)); // false
image.parentNode.removeChild(image);
console.log('undefined', (typeof image === 'undefined') ? true : false); // false ~ should be true?
console.log('null', (image === null) ? true : false); // false ~ should be true?
console.log('find-by-id', isNull(image)); // true ~ correct but there must be a better way than this?
};
</script>
</head>
<body>
<div id="demo"></div>
</body>
</html>
Basically what the above code demonstrates is an element being stored into a variable and then removed from dom.
(基本上上面代码演示的是一个元素存储到变量中然后从dom中删除。)
Even though the element has been removed from the dom, the variable retains the element as it was when first declared.(即使元素已从dom中删除，该变量仍保留元素，就像第一次声明时一样。)
In other words, it is not a live reference to the element itself, but rather a replica.(换句话说，它不是元素本身的实时引用，而是副本。)
As a result, checking the variable's value (the element) for existence will provide an unexpected result.(因此，检查变量的值（元素）是否存在将提供意外结果。)
The isNull function is my attempt to check for an elements existence from a variable, and it works, but I would like to know if there is an easier way to accomplish the same result.
(isNull函数是我尝试从变量中检查元素是否存在，并且它有效，但我想知道是否有更简单的方法来实现相同的结果。)
PS: I'm also interested in why JavaScript variables behave like this if anyone knows of some good articles related to the subject.
(PS：如果有人知道一些与该主题相关的好文章，我也对JavaScript变量的行为感兴趣。)
ask by Justin Bull translate from so
