Get array of object's keys

I would like to get the keys of a JavaScript object as an array, either in jQuery or pure JavaScript.

Is there a less verbose way than this?

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' };
var keys = [];
for (var key in foo) {
    keys.push(key);
}

Solution 1:

Use Object.keys:

var foo = {
  'alpha': 'puffin',
  'beta': 'beagle'
};

var keys = Object.keys(foo);
console.log(keys) // ['alpha', 'beta'] 
// (or maybe some other order, keys are unordered).

This is an ES5 feature. This means it works in all modern browsers but will not work in legacy browsers.

The ES5-shim has a implementation of Object.keys you can steal

Solution 2:

You can use jQuery's $.map.

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' },
keys = $.map(foo, function(v, i){
  return i;
});

Solution 3:

Of course, Object.keys() is the best way to get an Object's keys. If it's not available in your environment, it can be trivially shimmed using code such as in your example (except you'd need to take into account your loop will iterate over all properties up the prototype chain, unlike Object.keys()'s behaviour).

However, your example code...

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' };
var keys = [];
for (var key in foo) {
    keys.push(key);
}

jsFiddle.

...could be modified. You can do the assignment right in the variable part.

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' };
var keys = [], i = 0;
for (keys[i++] in foo) {}

jsFiddle.

Of course, this behaviour is different to what Object.keys() actually does (jsFiddle). You could simply use the shim on the MDN documentation.

Solution 4:

In case you're here looking for something to list the keys of an n-depth nested object as a flat array:

const getObjectKeys = (obj, prefix = '') => {
  return Object.entries(obj).reduce((collector, [key, val]) => {
    const newKeys = [ ...collector, prefix ? `${prefix}.${key}` : key ]
    if (Object.prototype.toString.call(val) === '[object Object]') {
      const newPrefix = prefix ? `${prefix}.${key}` : key
      const otherKeys = getObjectKeys(val, newPrefix)
      return [ ...newKeys, ...otherKeys ]
    }
    return newKeys
  }, [])
}

console.log(getObjectKeys({a: 1, b: 2, c: { d: 3, e: { f: 4 }}}))