Would assigning a member from a DOM element to a variable enable the variable to update itself?

For example if I assign

var n = document.getElementById('A').childNodes.length;

And then later append a child to A, would n update itself or would I ha开发者_如何学编程ve to assign it the new length again?

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No, it will not automatically update itself. The reason is that what you are doing is assigning the value of the length property, which is a number, to the variable n. Hence, n is not aware of the object property it came from, it merely stores a number. Primitive types in JavaScript are assigned/passed by value, whereas objects are passed by reference. This is why doing var o = document.getElementById('A'); would work in the manner you describe - what you're assigning to o is an object and not a primitive type.

Note: By "primitive type" I mean any of the following: Undefined, Null, Boolean, Number, or String

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No, you will have to re evaluate the variable.