Create array of unique objects by property

I'd probably use a flags object during the filtering, like this:

var flags = {};
var newPlaces = places.filter(function(entry) {
    if (flags[entry.city]) {
        return false;
    }
    flags[entry.city] = true;
    return true;
});

That uses Array#filter from ECMAScript5 (ES5), which is one of the ES5 additions that can be shimmed (search for "es5 shim" for several options).

You can do it without filter, of course, it's just a bit more verbose:

var flags = {};
var newPlaces = [];
var index;
for (index = 0; index < places.length; ++index) {
    if (!flags[entry.city]) {
        flags[entry.city] = true;
        newPlaces.push(entry);
    }
});

Both of the above assume the first object with a given city should be kept, and all other discarded.

Note: As user2736012 points out below, my test if (flags[entry.city]) will be true for cities with names that happen to be the same as properties that exist on Object.prototype such as toString. Very unlikely in this case, but there are four ways to avoid the possibility:

(My usual preferred solution) Create the object without a prototype: var flags = Object.create(null);. This is a feature of ES5. Note that this cannot be shimmed for obsolete browsers like IE8 (the single-argument version of Object.create can be except when that argument's value is null).
Use hasOwnProperty for the test, e.g. if (flags.hasOwnProperty(entry.city))
Put a prefix on that you know doesn't exist for any Object.prototype property, such as xx:
```
var key = "xx" + entry.city;
if (flags[key]) {
    // ...
}
flags[key] = true;
```

As of ES2015, you could use a Set instead:

const flags = new Set();
const newPlaces = places.filter(entry => {
    if (flags.has(entry.city)) {
        return false;
    }
    flags.add(entry.city);
    return true;
});

Shortest~~, but not best performance~~ (see update bellow) solution for es6 :

function unique(array, propertyName) {
   return array.filter((e, i) => array.findIndex(a => a[propertyName] === e[propertyName]) === i);
}

performance: https://jsperf.com/compare-unique-array-by-property

https://lodash.com/docs#uniqBy

https://github.com/lodash/lodash/blob/4.13.1/lodash.js#L7711

/**
 * This method is like `_.uniq` except that it accepts `iteratee` which is
 * invoked for each element in `array` to generate the criterion by which
 * uniqueness is computed. The iteratee is invoked with one argument: (value).
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Array
 * @param {Array} array The array to inspect.
 * @param {Array|Function|Object|string} [iteratee=_.identity]
 *  The iteratee invoked per element.
 * @returns {Array} Returns the new duplicate free array.
 * @example
 *
 * _.uniqBy([2.1, 1.2, 2.3], Math.floor);
 * // => [2.1, 1.2]
 *
 * // The `_.property` iteratee shorthand.
 * _.uniqBy([{ 'x': 1 }, { 'x': 2 }, { 'x': 1 }], 'x');
 * // => [{ 'x': 1 }, { 'x': 2 }]
 */

Create array of unique objects by property

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