Question

I need the index of the first value in the array, that matches a custom compare function.

The very nice underscorej has a “find” function that returns the first value where a function returns true, but I would need this that returns the index instead. Is there a version of indexOf available somewhere, where I can pass a function used to comparing?

Thanks for any suggestions!

Answer 1

Here’s the Underscore way to do it – this augments the core Underscore function with one that accepts an iterator function:

// save a reference to the core implementation
var indexOfValue = _.indexOf;

// using .mixin allows both wrapped and unwrapped calls:
// _(array).indexOf(...) and _.indexOf(array, ...)
_.mixin({

    // return the index of the first array element passing a test
    indexOf: function(array, test) {
        // delegate to standard indexOf if the test isn't a function
        if (!_.isFunction(test)) return indexOfValue(array, test);
        // otherwise, look for the index
        for (var x = 0; x < array.length; x++) {
            if (test(array[x])) return x;
        }
        // not found, return fail value
        return -1;
    }

});

_.indexOf([1,2,3], 3); // 2
_.indexOf([1,2,3], function(el) { return el > 2; } ); // 2