Question

Have a look at this code sample or go to the jsfiddle

function printRelation(a, b, out) {
    var text;
    if (a === b) {
        text = "a === b";
    } else if (a == b) {
        text = "a == b";
    } else {
        text = "a != b";
    }
    $('#' + out).text(text);
}

var a = [0, 0, 2], b = a;
printRelation(a, b, 'out1');
a = [0, 0, 2];
b = [0, 0, 2];
printRelation(a, b, 'out2');

I would have expected both tests to output a === b, but only the first one does. The second one outputs a != b. Can anyone explain this behaviour? How can I efficiently compare arrays in javascript?

Answer 1

JavaScript array comparisons like you’ve written are just simple object reference comparisons. They’re not “deep” comparisons element by element.

You can write your own comparison function, something to check that the lengths are the same and the the elements are the same, either in order or not, as fits your needs. edit As others point out, there are several libraries that include array comparison functions. If you find one that meets your definition of “equality” (not unlikely), and you don’t mind considering incorporating that library into your architecture, that might be a good idea.