uniquify an array of objects on some key

uniquify-on.js

function uniquifyOn(arr, fn) {

    var returnArr = [],
        tempArr = [];

    arr.forEach(function (item) {
        var key = fn(item);
        if ( tempArr.indexOf(key) === -1) {
            returnArr.push(item);
            tempArr.push(key);
        }
    });

    return returnArr;
}

// Example Usage
var cities = [{name: 'Austin', index: 0},{name: 'Chattanooga', index: 1},{name: 'Austin', index: 2},{name: 'Atlanta', index: 3}];

console.log(uniquifyOn(cities, function (item) {
    return item.name;
}));

// returns
//  [ { name: 'Austin', index: 0 },
//    { name: 'Chattanooga', index: 1 },
//    { name: 'Atlanta', index: 3 } ]