test-unique-by-prop.js

var uniqueByProp = require("./unique-by-prop"),
    assert = require("assert");

describe('Unique by prop', function () {
    it('filter values based on identical property', function () {
        var arr = [{x: 1}, {x: 1}, {x: 2}];

        assert.deepEqual(uniqueByProp(arr, 'x'), [{x: 1}, {x: 2}]);
    });

    it('filter values based on identical property in the presence of other properties', function () {
        var arr = [{x: 1, y: 1}, {x: 1, y: 2}, {x: 2, y: 3}];

        assert.deepEqual(uniqueByProp(arr, 'x'), [{x: 1, y: 1}, {x: 2, y: 3}]);
    });
});

unique-by-prop.js

/**
 * Function that filters an array of objects by checking duplicates of
 * a particular property.  For example
 *
 * [{x: 1}, {x: 1}, {x: 2}]
 *
 * We can use this function (array, 'x') to get back
 * [{x: 1}, {x: 2}]
 */
module.exports = function (array, property) {
    var foundHash = {};

    return array.filter(function (item) {
        var prop = item[property];

        // we already found a matching item with this property
        if (foundHash[prop]) {
            // filters out
            return false;
        }

        // did not find an item with this property yet, so we record it
        foundHash[prop] = true;

        // keeps
        return true;
    });
}