What is the code " length === +length" mean in JavaScript?

Question

I just read the underscope source code, and cannot get the point from this code:

_.each = _.forEach = function(obj, iterator, context) {
    if (obj == null) return obj;
    iterator = createCallback(iterator, context);
    var i, length = obj.length;
    if (length === +length) {   // why +length?
        for (i = 0; i < length; i++) {
            iterator(obj[i], i, obj);
        }
    } else {
        var keys = _.keys(obj);
        for (i = 0, length = keys.length; i < length; i++) {
            iterator(obj[keys[i]], keys[i], obj);
        }
    }
    return obj;
};

why length===+length ? I guess this used for force to convert if length is not a number? Could somebody give me a hand?

There was an exact duplicate, it's even related to the exact same code : http://stackoverflow.com/questions/8330499/operator-before-expression-in-javascript-what-does-it-do — Denys Séguret, Jul 24 '14 at 07:14

score 5 · Accepted Answer · edited May 23 '17 at 12:14

5

+length is a method to convert anything to a number.

If it's a number, the value doesn't change, and the comparison returns true. If it's not a number, the assertion is false.

What is unary + used for in Javascript?

edited May 23 '17 at 12:14

Community

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answered Jul 24 '14 at 07:12

StingRay21

342
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score 2 · Answer 2 · answered Jul 24 '14 at 07:10

2

+length converts any value of length to a number (NaN if not possible).

So length===+length just tests that length is really a number (not a string that could be converted to a number), and that it's not NaN (which isn't equal to itself).

answered Jul 24 '14 at 07:10

Denys Séguret

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What is the code " length === +length" mean in JavaScript?

2 Answers2