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

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?

Answer 1

+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?

Answer 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).