map function for objects (instead of arrays)
There is no native map to the Object object, but how about this:
var myObject = { 'a': 1, 'b': 2, 'c': 3 };Object.keys(myObject).map(function(key, index) { myObject[key] *= 2;});console.log(myObject);// => { 'a': 2, 'b': 4, 'c': 6 }But you could easily iterate over an object using for ... in:
var myObject = { 'a': 1, 'b': 2, 'c': 3 };for (var key in myObject) { if (myObject.hasOwnProperty(key)) { myObject[key] *= 2; }}console.log(myObject);// { 'a': 2, 'b': 4, 'c': 6 }Update
A lot of people are mentioning that the previous methods do not return a new object, but rather operate on the object itself. For that matter I wanted to add another solution that returns a new object and leaves the original object as it is:
var myObject = { 'a': 1, 'b': 2, 'c': 3 };// returns a new object with the values at each key mapped using mapFn(value)function objectMap(object, mapFn) { return Object.keys(object).reduce(function(result, key) { result[key] = mapFn(object[key]) return result }, {})}var newObject = objectMap(myObject, function(value) { return value * 2})console.log(newObject);// => { 'a': 2, 'b': 4, 'c': 6 }console.log(myObject);// => { 'a': 1, 'b': 2, 'c': 3 }Array.prototype.reduce reduces an array to a single value by somewhat merging the previous value with the current. The chain is initialized by an empty object {}. On every iteration a new key of myObject is added with twice the key as the value.
Update
With new ES6 features, there is a more elegant way to express objectMap.
const objectMap = (obj, fn) => Object.fromEntries( Object.entries(obj).map( ([k, v], i) => [k, fn(v, k, i)] ) ) const myObject = { a: 1, b: 2, c: 3 }console.log(objectMap(myObject, v => 2 * v))
How about a one-liner in JS ES10 / ES2019 ?
Making use of Object.entries() and Object.fromEntries():
let newObj = Object.fromEntries(Object.entries(obj).map(([k, v]) => [k, v * v]));The same thing written as a function:
function objMap(obj, func) { return Object.fromEntries(Object.entries(obj).map(([k, v]) => [k, func(v)]));}// To square each value you can call it like this:let mappedObj = objMap(obj, (x) => x * x);This function uses recursion to square nested objects as well:
function objMap(obj, func) { return Object.fromEntries( Object.entries(obj).map(([k, v]) => [k, v === Object(v) ? objMap(v, func) : func(v)] ) );}// To square each value you can call it like this:let mappedObj = objMap(obj, (x) => x * x);With ES7 / ES2016 you cant' use Objects.fromEntries, but you can achieve the same using Object.assign in combination with spread operators and computed key names syntax:
let newObj = Object.assign(...Object.entries(obj).map(([k, v]) => ({[k]: v * v})));ES6 / ES2015 Doesn't allow Object.entries, but you could use Object.keys instead:
let newObj = Object.assign(...Object.keys(obj).map(k => ({[k]: obj[k] * obj[k]})));ES6 also introduced for...of loops, which allow a more imperative style:
let newObj = {}for (let [k, v] of Object.entries(obj)) { newObj[k] = v * v;}array.reduce()
Instead of Object.fromEntries and Object.assign you can also use reduce for this:
let newObj = Object.entries(obj).reduce((p, [k, v]) => ({ ...p, [k]: v * v }), {});Inherited properties and the prototype chain:
In some rare situation you may need to map a class-like object which holds properties of an inherited object on its prototype-chain. In such cases Object.keys() and Object.entries() won't work, because these functions do not include the prototype chain.
If you need to map inherited properties, you can use for (key in myObj) {...}.
Here is an example of such situation:
const obj1 = { 'a': 1, 'b': 2, 'c': 3}const obj2 = Object.create(obj1); // One of multiple ways to inherit an object in JS.// Here you see how the properties of obj1 sit on the 'prototype' of obj2console.log(obj2) // Prints: obj2.__proto__ = { 'a': 1, 'b': 2, 'c': 3}console.log(Object.keys(obj2)); // Prints: an empty Array.console.log(Object.entries(obj2)); // Prints: an empty Array.for (let key in obj2) { console.log(key); // Prints: 'a', 'b', 'c'}However, please do me a favor and avoid inheritance. :-)
No native methods, but lodash#mapValues will do the job brilliantly
_.mapValues({ 'a': 1, 'b': 2, 'c': 3} , function(num) { return num * 3; });// → { 'a': 3, 'b': 6, 'c': 9 }