CODEWITHSUNDEEP
javascriptecmascript-5
Richard
Richard

Reputation: 32909

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);
}

Upvotes: 516

Views: 655434

Answers (8)

Ondra Žižka
Ondra Žižka

Reputation: 46806

Year 2022 and JavaScript still does not have a sound way to work with hashes?

This issues a warning but works:

Object.prototype.keys = function() { return Object.keys(this) }
console.log("Keys of an object: ", { a:1, b:2 }.keys() )

// Keys of an object:  Array [ "a", "b" ]
// WARN: Line 8:1:  Object prototype is read only, properties should not be added  no-extend-native

That said, Extending Built-in Objects is Controversial.

Upvotes: 2

Raynos
Raynos

Reputation: 169401

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

Upvotes: 811

Cody Moniz
Cody Moniz

Reputation: 5065

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 }}}))

Upvotes: 18

mohammad javad ahmadi
mohammad javad ahmadi

Reputation: 2081

If you decide to use Underscore.js you better do

var foo = { 'alpha' : 'puffin', 'beta' : 'beagle' };
var keys = [];
_.each( foo, function( val, key ) {
    keys.push(key);
});
console.log(keys);

Upvotes: 0

Willem van der Veen
Willem van der Veen

Reputation: 36620

Summary

For getting all of the keys of an Object you can use Object.keys(). Object.keys() takes an object as an argument and returns an array of all the keys.

Example:

const object = {
  a: 'string1',
  b: 42,
  c: 34
};

const keys = Object.keys(object)

console.log(keys);

console.log(keys.length) // we can easily access the total amount of properties the object has

In the above example we store an array of keys in the keys const. We then can easily access the amount of properties on the object by checking the length of the keys array.

Getting the values with: Object.values()

The complementary function of Object.keys() is Object.values(). This function takes an object as an argument and returns an array of values. For example:

const object = {
  a: 'random',
  b: 22,
  c: true
};


console.log(Object.values(object));

Upvotes: 5

alex
alex

Reputation: 490263

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.

Upvotes: 40

Dexygen
Dexygen

Reputation: 12561

I don't know about less verbose but I was inspired to coerce the following onto one line by the one-liner request, don't know how Pythonic it is though ;)

var keys = (function(o){var ks=[]; for(var k in o) ks.push(k); return ks})(foo);

Upvotes: 8

gen_Eric
gen_Eric

Reputation: 227260

You can use jQuery's $.map.

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

Upvotes: 62

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