增强型Java语句安全使用吗？

Question

Using the jQuery for-loop is quite slow, which is the reason why I'm considering using the regular for-statement more often. In order to have direct access to the current element, I found the following syntax (for regular arrays, not for objects of course):

for (var i = 0, e; e = array[i]; i++) { ... }

where e in the loop represents the current element.

Is this syntax safe to use across all browsers?

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Addition

OK, I guess this could work, but it is not so useful anymore for a short notation:

for (var i = 0, e; (e = array[i]) !== void(0); i++) { ... }

Thank you all for answering!

Answer 1

That is not a very good loop. Consider this array:

var array = [0, 1, 2, 3];

It would stop on the first element because 0 is a falsey value. Same with

var array = ["foo", "bar", false, "hello"];

It would only get to "foo" and "bar"

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Consider using this loop

for (var i=0, len=array.length; i<len; i++) { ... }

It works everywhere, only calculates array.length once, and is plenty performant.

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Per TJ's comment, the scope of the args i and len above will exist your current function. So be careful that you don't make variable conflicts.

A somewhat common (but clunky) way of safeguarding against this is prefixing vars with _ . Like this

for (var _i=0, _len=array.length; _i<_len; _i++) { ... }

Answer 2

As naomik points out , that form of loop will break if any of the array elements has a falsey value. Falsey values are false , null , undefined , "" , 0 , and NaN . So it would work, for instance, for an array of non-null object references. Not so much for an array of strings or numbers.

But if your question is about the syntax , then yes, it's "safe" in that it will work (and fail on falsey elements) in all JavaScript engines. The key bits you're relying on are:

1. That accessing the element beyond the end of the array (eg, at array[array.length] ) will give you a falsey value ( undefined ) rather than throwing an exception, and

2. That the result of an assignment expression ( e = array[i] ) is the value that was assigned.

Yes, both of those are reliable. (Well, #1 is reliable if the array really is a JavaScript array. Host-provided array-like objects may vary.)

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In any case, note that neither i nor e is scoped only to the loop. ES6 will have let , which will be, but variables declared with var are scoped to the function they're in.

Answer 3

I do not recommend that. You see, if your array looks like this for example:

array = ["lala", 078, false, 992, "kas"];

Then your loop would only go through the first two, since the term e = array[i]; would return false, because the third entry in the array is literally false. This is better:

for (var i = 0, e; (e = array[i])===undefined; i++) { ... }

Make sure no one overwrites the undefined variable, eg by using a closure: How does this JavaScript/JQuery Syntax work: (function( window, undefined ) { })(window)?