map function for objects (instead of arrays)


I have an object:

myObject = { 'a': 1, 'b': 2, 'c': 3 }

I am looking for a native method, similar to Array.prototype.map that would be used as follows:

newObject = myObject.map(function (value, label) {
    return value * value;
});

// newObject is now { 'a': 1, 'b': 4, 'c': 9 }

Does JavaScript have such a map function for objects? (I want this for Node.JS, so I don't care about cross-browser issues.)

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 with immediate variable assignment in plain JS (ES6 / ES2015) ?

Making use of spread operator and computed key name syntax:

let newObj = Object.assign({}, ...Object.keys(obj).map(k => ({[k]: obj[k] * obj[k]})));

jsbin

Another version using reduce:

let newObj = Object.keys(obj).reduce((p, c) => ({...p, [c]: obj[c] * obj[c]}), {});

jsbin

First example as a function:

const oMap = (o, f) => Object.assign({}, ...Object.keys(o).map(k => ({ [k]: f(o[k]) })));

// To square each value you can call it like this:
let mappedObj = oMap(myObj, (x) => x * x);

jsbin

If you want to map a nested object recursively in a functional style, it can be done like this:

const sqrObjRecursive = obj =>
  Object.keys(obj).reduce(
    (newObj, key) =>
      obj[key] && typeof obj[key] === "object"
        ? { ...newObj, [key]: sqrObjRecursive(obj[key]) } // recurse.
        : { ...newObj, [key]: obj[key] * obj[key] }, // square val.
    {}
  );       

jsbin

Or more imperatively, like this:

const sqrObjRecursive = obj => {
  Object.keys(obj).forEach(key => {
    if (typeof obj[key] === "object") obj[key] = sqrObjRecursive(obj[key]);
    else obj[key] = obj[key] * obj[key];
  });
  return obj;
};

jsbin

Since ES7 / ES2016 you can use Object.entries() instead of Object.keys() e.g. like this:

let newObj = Object.assign({}, ...Object.entries(obj).map(([k, v]) => ({[k]: v * v})));

ES2019 introduced Object.fromEntries(), which simplifies this even more:

let newObj = Object.fromEntries(Object.entries(obj).map(([k, v]) => [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() won't work, because Object.keys() does not enumerate inherited properties. If you need to map inherited properties, you should use for (key in myObj) {...}.

Here is an example of an object which inherits the properties of another object and how Object.keys() doesn't work in such scenario.

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 obj2
console.log(obj2)  // Prints: obj2.__proto__ = { 'a': 1, 'b': 2, 'c': 3}

console.log(Object.keys(obj2));  // Prints: an empty Array.

for (key in obj2) {
  console.log(key);              // Prints: 'a', 'b', 'c'
}

jsbin

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 }

It's pretty easy to write one:

Object.map = function(o, f, ctx) {
    ctx = ctx || this;
    var result = {};
    Object.keys(o).forEach(function(k) {
        result[k] = f.call(ctx, o[k], k, o); 
    });
    return result;
}

with example code:

> o = { a: 1, b: 2, c: 3 };
> r = Object.map(o, function(v, k, o) {
     return v * v;
  });
> r
{ a : 1, b: 4, c: 9 }

NB: this version also allows you to (optionally) set the this context for the callback, just like the Array method.

EDIT - changed to remove use of Object.prototype, to ensure that it doesn't clash with any existing property named map on the object.


You could use Object.keys and then forEach over the returned array of keys:

var myObject = { 'a': 1, 'b': 2, 'c': 3 },
    newObject = {};
Object.keys(myObject).forEach(function (key) {
    var value = myObject[key];
    newObject[key] = value * value;
});

Or in a more modular fashion:

function map(obj, callback) {
    var result = {};
    Object.keys(obj).forEach(function (key) {
        result[key] = callback.call(obj, obj[key], key, obj);
    });
    return result;
}

newObject = map(myObject, function(x) { return x * x; });

Note that Object.keys returns an array containing only the object's own enumerable properties, thus it behaves like a for..in loop with a hasOwnProperty check.


This is straight bs, and everyone in the JS community knows it. There should be this functionality:

const obj1 = {a:4, b:7};
const obj2 = Object.map(obj1, (k,v) => v + 5);

console.log(obj1); // {a:4, b:7}
console.log(obj2); // {a:9, b:12}

here is the naïve implementation:

Object.map = function(obj, fn, ctx){

    const ret = {};

    for(let k of Object.keys(obj)){
        ret[k] = fn.call(ctx || null, k, obj[k]);
    });

    return ret;
};

it is super annoying to have to implement this yourself all the time ;)

If you want something a little more sophisticated, that doesn't interfere with the Object class, try this:

let map = function (obj, fn, ctx) {
  return Object.keys(obj).reduce((a, b) => {
    a[b] = fn.call(ctx || null, b, obj[b]);
    return a;
  }, {});
};


const x = map({a: 2, b: 4}, (k,v) => {
    return v*2;
});

but it is safe to add this map function to Object, just don't add to Object.prototype.

Object.map = ... // fairly safe
Object.prototype.map ... // not ok

I came here looking to find and answer for mapping an object to an array and got this page as a result. In case you came here looking for the same answer I was, here is how you can map and object to an array.

You can use map to return a new array from the object like so:

var newObject = Object.keys(myObject).map(function(key) {
   return myObject[key];
});

The accepted answer has two drawbacks:

  • It misuses Array.prototype.reduce, because reducing means to change the structure of a composite type, which doesn't happen in this case.
  • It is not particularly reusable

An ES6/ES2015 functional approach

Please note that all functions are defined in curried form.

// small, reusable auxiliary functions

const keys = o => Object.keys(o);

const assign = (...o) => Object.assign({}, ...o);

const map = f => xs => xs.map(x => f(x));

const mul = y => x => x * y;

const sqr = x => mul(x) (x);


// the actual map function

const omap = f => o => {
  o = assign(o); // A
  map(x => o[x] = f(o[x])) (keys(o)); // B
  return o;
};


// mock data

const o = {"a":1, "b":2, "c":3};


// and run

console.log(omap(sqr) (o));
console.log(omap(mul(10)) (o));

  • In line A o is reassigned. Since Javascript passes reference values by sharing, a shallow copy of o is generated. We are now able to mutate o within omap without mutating o in the parent scope.
  • In line B map's return value is ignored, because map performs a mutation of o. Since this side effect remains within omap and isn't visible in the parent scope, it is totally acceptable.

This is not the fastest solution, but a declarative and reusable one. Here is the same implementation as a one-line, succinct but less readable:

const omap = f => o => (o = assign(o), map(x => o[x] = f(o[x])) (keys(o)), o);

Addendum - why are objects not iterable by default?

ES2015 specified the iterator and iterable protocols. But objects are still not iterable and thus not mappable. The reason is the mixing of data and program level.


JavaScript just got the new Object.fromEntries method.

Example

function mapObject (obj, fn) {
  return Object.fromEntries(
    Object
      .entries(obj)
      .map(fn)
  )
}

const myObject = { a: 1, b: 2, c: 3 }
const myNewObject = mapObject(myObject, ([key, value]) => ([key, value * value]))
console.log(myNewObject)

Explanation

The code above converts the Object into an nested Array ([[<key>,<value>], ...]) wich you can map over. Object.fromEntries converts the Array back to an Object.

The cool thing about this pattern, is that you can now easily take object keys into account while mapping.

Documentation

Browser Support

Object.fromEntries is currently only supported by these browsers/engines, nevertheless there are polyfills available (e.g @babel/polyfill).


For maximum performance.

If your object doesn't change often but needs to be iterated on often I suggest using a native Map as a cache.

// example object
var obj = {a: 1, b: 2, c: 'something'};

// caching map
var objMap = new Map(Object.entries(obj));

// fast iteration on Map object
objMap.forEach((item, key) => {
  // do something with an item
  console.log(key, item);
});

Object.entries already works in Chrome, Edge, Firefox and beta Opera so it's a future-proof feature. It's from ES7 so polyfill it https://github.com/es-shims/Object.entries for IE where it doesn't work.


Minimal version (es6):

Object.entries(obj).reduce((a, [k, v]) => (a[k] = v * v, a), {})

you can use map method and forEach on arrays but if you want to use it on Object then you can use it with twist like below:

Using Javascript (ES6)

var obj = { 'a': 2, 'b': 4, 'c': 6 };   
Object.entries(obj).map( v => obj[v[0]] *= v[1] );
console.log(obj); //it will log as {a: 4, b: 16, c: 36}

var obj2 = { 'a': 4, 'b': 8, 'c': 10 };
Object.entries(obj2).forEach( v => obj2[v[0]] *= v[1] );
console.log(obj2); //it will log as {a: 16, b: 64, c: 100}

Using jQuery

var ob = { 'a': 2, 'b': 4, 'c': 6 };
$.map(ob, function (val, key) {
   ob[key] *= val;
});
console.log(ob) //it will log as {a: 4, b: 16, c: 36}

Or you can use other loops also like $.each method as below example:

$.each(ob,function (key, value) {
  ob[key] *= value;
});
console.log(ob) //it will also log as {a: 4, b: 16, c: 36}

The map function does not exist on the Object.prototype however you can emulate it like so

var myMap = function ( obj, callback ) {

    var result = {};

    for ( var key in obj ) {
        if ( Object.prototype.hasOwnProperty.call( obj, key ) ) {
            if ( typeof callback === 'function' ) {
                result[ key ] = callback.call( obj, obj[ key ], key, obj );
            }
        }
    }

    return result;

};

var myObject = { 'a': 1, 'b': 2, 'c': 3 };

var newObject = myMap( myObject, function ( value, key ) {
    return value * value;
});

I came upon this as a first-item in a Google search trying to learn to do this, and thought I would share for other folsk finding this recently the solution I found, which uses the npm package immutable.

I think its interesting to share because immutable uses the OP's EXACT situation in their own documentation - the following is not my own code but pulled from the current immutable-js documentation:

const { Seq } = require('immutable')
const myObject = { a: 1, b: 2, c: 3 }
Seq(myObject).map(x => x * x).toObject();
// { a: 1, b: 4, c: 9 } 

Not that Seq has other properties ("Seq describes a lazy operation, allowing them to efficiently chain use of all the higher-order collection methods (such as map and filter) by not creating intermediate collections") and that some other immutable-js data structures might also do the job quite efficiently.

Anyone using this method will of course have to npm install immutable and might want to read the docs:

https://facebook.github.io/immutable-js/


EDIT: The canonical way using newer JavaScript features is -

const identity = x =>
  x

const omap = (f = identity, o = {}) =>
  Object.fromEntries(
    Object.entries(o).map(([ k, v ]) =>
      [ k, f(v) ]
    )
  )

Where o is some object and f is your mapping function. Or we could say, given a function from a -> b, and an object with values of type a, produce an object with values of type b. As a pseudo type signature -

// omap : (a -> b, { a }) -> { b }

The original answer was written to demonstrate a powerful combinator, mapReduce which allows us to think of our transformation in a different way

  1. m, the mapping function – gives you a chance to transform the incoming element before…
  2. r, the reducing function – this function combines the accumulator with the result of the mapped element

Intuitively, mapReduce creates a new reducer we can plug directly into Array.prototype.reduce. But more importantly, we can implement our object functor implementation omap plainly by utilizing the object monoid, Object.assign and {}.

const identity = x =>
  x
  
const mapReduce = (m, r) =>
  (a, x) => r (a, m (x))

const omap = (f = identity, o = {}) =>
  Object
    .keys (o)
    .reduce
      ( mapReduce
          ( k => ({ [k]: f (o[k]) })
          , Object.assign
          )
      , {}
      )
          
const square = x =>
  x * x
  
const data =
  { a : 1, b : 2, c : 3 }
  
console .log (omap (square, data))
// { a : 1, b : 4, c : 9 }

Notice the only part of the program we actually had to write is the mapping implementation itself –

k => ({ [k]: f (o[k]) })

Which says, given a known object o and some key k, construct an object and whose computed property k is the result of calling f on the key's value, o[k].

We get a glimpse of mapReduce's sequencing potential if we first abstract oreduce

// oreduce : (string * a -> string * b, b, { a }) -> { b }
const oreduce = (f = identity, r = null, o = {}) =>
  Object
    .keys (o)
    .reduce
      ( mapReduce
          ( k => [ k, o[k] ]
          , f
          )
      , r
      )

// omap : (a -> b, {a}) -> {b}
const omap = (f = identity, o = {}) =>
  oreduce
    ( mapReduce
        ( ([ k, v ]) =>
            ({ [k]: f (v) })
        , Object.assign
        )
    , {}
    , o
    )

Everything works the same, but omap can be defined at a higher-level now. Of course the new Object.entries makes this look silly, but the exercise is still important to the learner.

You won't see the full potential of mapReduce here, but I share this answer because it's interesting to see just how many places it can be applied. If you're interested in how it is derived and other ways it could be useful, please see this answer.


Based on @Amberlamps answer, here's a utility function (as a comment it looked ugly)

function mapObject(obj, mapFunc){
    return Object.keys(obj).reduce(function(newObj, value) {
        newObj[value] = mapFunc(obj[value]);
        return newObj;
    }, {});
}

and the use is:

var obj = {a:1, b:3, c:5}
function double(x){return x * 2}

var newObj = mapObject(obj, double);
//=>  {a: 2, b: 6, c: 10}

var myObject = { 'a': 1, 'b': 2, 'c': 3 };


Object.prototype.map = function(fn){
    var oReturn = {};
    for (sCurObjectPropertyName in this) {
        oReturn[sCurObjectPropertyName] = fn(this[sCurObjectPropertyName], sCurObjectPropertyName);
    }
    return oReturn;
}
Object.defineProperty(Object.prototype,'map',{enumerable:false});





newObject = myObject.map(function (value, label) {
    return value * value;
});


// newObject is now { 'a': 1, 'b': 4, 'c': 9 }

My response is largely based off the highest rated response here and hopefully everyone understands (have the same explanation on my GitHub, too). This is why his impementation with map works:

Object.keys(images).map((key) => images[key] = 'url(' + '"' + images[key] + '"' +    
')');

The purpose of the function is to take an object and modify the original contents of the object using a method available to all objects (objects and arrays alike) without returning an array. Almost everything within JS is an object, and for that reason elements further down the pipeline of inheritance can potentially technically use those available to those up the line (and the reverse it appears).

The reason that this works is due to the .map functions returning an array REQUIRING that you provide an explicit or implicit RETURN of an array instead of simply modifying an existing object. You essentially trick the program into thinking the object is an array by using Object.keys which will allow you to use the map function with its acting on the values the individual keys are associated with (I actually accidentally returned arrays but fixed it). As long as there isn't a return in the normal sense, there will be no array created with the original object stil intact and modified as programmed.

This particular program takes an object called images and takes the values of its keys and appends url tags for use within another function. Original is this:

var images = { 
snow: 'https://www.trbimg.com/img-5aa059f5/turbine/bs-md-weather-20180305', 
sunny: 'http://www.cubaweather.org/images/weather-photos/large/Sunny-morning-east-   
Matanzas-city- Cuba-20170131-1080.jpg', 
rain: 'https://i.pinimg.com/originals/23/d8
/ab/23d8ab1eebc72a123cebc80ce32b43d8.jpg' };

...and modified is this:

var images = { 
snow: url('https://www.trbimg.com/img-5aa059f5/turbine/bs-md-weather-20180305'),     
sunny: url('http://www.cubaweather.org/images/weather-photos/large/Sunny-morning-   
east-Matanzas-city- Cuba-20170131-1080.jpg'), 
rain: url('https://i.pinimg.com/originals/23/d8
/ab/23d8ab1eebc72a123cebc80ce32b43d8.jpg') 
};

The object's original structure is left intact allowing for normal property access as long as there isn't a return. Do NOT have it return an array like normal and everything will be fine. The goal is REASSIGNING the original values (images[key]) to what is wanted and not anything else. As far as I know, in order to prevent array output there HAS to be REASSIGNMENT of images[key] and no implicit or explicit request to return an array (variable assignment does this and was glitching back and forth for me).

EDIT:

Going to address his other method regarding new object creation to avoid modifying original object (and reassignment appears to still be necessary in order to avoid accidentally creating an array as output). These functions use arrow syntax and are if you simply want to create a new object for future use.

const mapper = (obj, mapFn) => Object.keys(obj).reduce((result, key) => {
                result[key] = mapFn(obj)[key];
                return result;
            }, {});

var newImages = mapper(images, (value) => value);

The way these functions work is like so:

mapFn takes the function to be added later (in this case (value) => value) and simply returns whatever is stored there as a value for that key (or multiplied by two if you change the return value like he did) in mapFn(obj)[key],

and then redefines the original value associated with the key in result[key] = mapFn(obj)[key]

and returns the operation performed on result (the accumulator located in the brackets initiated at the end of the .reduce function).

All of this is being performed on the chosen object and STILL there CANNOT be an implicit request for a returned array and only works when reassigning values as far as I can tell. This requires some mental gymnastics but reduces the lines of code needed as can be seen above. Output is exactly the same as can be seen below:

{snow: "https://www.trbimg.com/img-5aa059f5/turbine/bs-   
md-weather-20180305", sunny: "http://www.cubaweather.org/images/weather-
photos/l…morning-east-Matanzas-city-Cuba-20170131-1080.jpg", rain: 
"https://i.pinimg.com/originals/23/d8
/ab/23d8ab1eebc72a123cebc80ce32b43d8.jpg"}

Keep in mind this worked with NON-NUMBERS. You CAN duplicate ANY object by SIMPLY RETURNING THE VALUE in the mapFN function.


The first function answers the question. It creates a map.

The second function doesn't create a map. Instead, it loops through properties in the existing object using hasOwnProperty(). This map alternative may be a better solution in some situations.

var myObject = { 'a': 1, 'b': 2, 'c': 3 }


//creates a map (answers question, but function below is 
//much better in some situations)
var newObject = {};
makeMap();    
function makeMap() {
    for (var k in myObject) {
        var value = myObject[k];
        newObject[k] = value * value;
    }
    console.log(newObject); //mapped array
}


//Doesn't create a map, just applies the function to
//a specific property using existing data
function getValue(key) {
  for (var k in myObject) {
    if (myObject.hasOwnProperty(key)) {
      var value = myObject[key]
      return value * value; //stops iteration
    }
  }
}
Input: <input id="input" value="" placeholder="a, b or c"><br>
Output:<input id="output"><br>
<button onclick="output.value=getValue(input.value)" >Get value</button>


If you're interested in mapping not only values but also keys, I have written Object.map(valueMapper, keyMapper), that behaves this way:

var source = { a: 1, b: 2 };
function sum(x) { return x + x }

source.map(sum);            // returns { a: 2, b: 4 }
source.map(undefined, sum); // returns { aa: 1, bb: 2 }
source.map(sum, sum);       // returns { aa: 2, bb: 4 }

I needed a version that allowed modifying the keys as well (based on @Amberlamps and @yonatanmn answers);

var facts = [ // can be an object or array - see jsfiddle below
    {uuid:"asdfasdf",color:"red"},
    {uuid:"sdfgsdfg",color:"green"},
    {uuid:"dfghdfgh",color:"blue"}
];

var factObject = mapObject({}, facts, function(key, item) {
    return [item.uuid, {test:item.color, oldKey:key}];
});

function mapObject(empty, obj, mapFunc){
    return Object.keys(obj).reduce(function(newObj, key) {
        var kvPair = mapFunc(key, obj[key]);
        newObj[kvPair[0]] = kvPair[1];
        return newObj;
    }, empty);
}

factObject=

{
"asdfasdf": {"color":"red","oldKey":"0"},
"sdfgsdfg": {"color":"green","oldKey":"1"},
"dfghdfgh": {"color":"blue","oldKey":"2"}
}

Edit: slight change to pass in the starting object {}. Allows it to be [] (if the keys are integers)


To responds more closely to what precisely the OP asked for, the OP wants an object:

myObject = { 'a': 1, 'b': 2, 'c': 3 }

to have a map method myObject.map,

similar to Array.prototype.map that would be used as follows:

newObject = myObject.map(function (value, label) {
    return value * value;
});
// newObject is now { 'a': 1, 'b': 4, 'c': 9 }

The imho best (measured in terms to "close to what is asked" + "no ES{5,6,7} required needlessly") answer would be:

myObject.map = function mapForObject(callback)
{
  var result = {};
  for(var property in this){
    if(this.hasOwnProperty(property) && property != "map"){
      result[property] = callback(this[property],property,this);
    }
  }
  return result;
}

The code above avoids intentionally using any language features, only available in recent ECMAScript editions. With the code above the problem can be solved lke this:

myObject = { 'a': 1, 'b': 2, 'c': 3 };

myObject.map = function mapForObject(callback)
{
  var result = {};
  for(var property in this){
    if(this.hasOwnProperty(property) && property != "map"){
      result[property] = callback(this[property],property,this);
    }
  }
  return result;
}

newObject = myObject.map(function (value, label) {
  return value * value;
});
console.log("newObject is now",newObject);
alternative test code here

Besides frowned upon by some, it would be a possibility to insert the solution in the prototype chain like this.

Object.prototype.map = function(callback)
{
  var result = {};
  for(var property in this){
    if(this.hasOwnProperty(property)){
      result[property] = callback(this[property],property,this);
    }
  }
  return result;
}

Something, which when done with careful oversight should not have any ill effects and not impact map method of other objects (i.e. Array's map).


First, convert your HTMLCollection using Object.entries(collection). Then it’s an iterable you can now use the .map method on it.

Object.entries(collection).map(...)

reference https://medium.com/@js_tut/calling-javascript-code-on-multiple-div-elements-without-the-id-attribute-97ff6a50f31


Define a function mapEntries.

mapEntries takes a callback function that is called on each entry in the object with the parameters value, key and object. It should return the a new value.

mapEntries should return a new object with the new values returned from the callback.

Object.defineProperty(Object.prototype, 'mapEntries', {
  enumerable: false,
  value: function (mapEntriesCallback) {
    return Object.fromEntries(
      Object.entries(this).map(
        ([key, value]) => [key, mapEntriesCallback(value, key, this)]
      )
    )
  }
})


// Usage example:

var object = {a: 1, b: 2, c: 3}
var newObject = object.mapEntries(value => value * value)
console.log(newObject)
//> {a: 1, b: 4, c: 9}

Edit: A previous version didn't specify that this is not an enumerable property


Hey wrote a little mapper function that might help.

    function propertyMapper(object, src){
         for (var property in object) {   
           for (var sourceProp in src) {
               if(property === sourceProp){
                 if(Object.prototype.toString.call( property ) === '[object Array]'){
                   propertyMapper(object[property], src[sourceProp]);
                   }else{
                   object[property] = src[sourceProp];
                }
              }
            }
         }
      }

I specifically wanted to use the same function that I was using for arrays for a single object, and wanted to keep it simple. This worked for me:

var mapped = [item].map(myMapFunction).pop();

A different take on it is to use a custom json stringify function that can also work on deep objects. This might be useful if you intend to post it to the server anyway as json

const obj = { 'a': 1, 'b': 2, x: {'c': 3 }}
const json = JSON.stringify(obj, (k, v) => typeof v === 'number' ? v * v : v)

console.log(json)
console.log('back to json:', JSON.parse(json))


I handle only strings to reduce exemptions:

Object.keys(params).map(k => typeof params[k] == "string" ? params[k] = params[k].trim() : null);

Object Mapper in TypeScript

I like the examples that use Object.fromEntries such as this one, but still, they are not very easy to use. The answers that use Object.keys and then look up the key are actually doing multiple look-ups that may not be necessary.

I wished there was an Object.map function, but we can create our own and call it objectMap with the ability to modify both key and value:

Usage (JavaScript):

const myObject = { 'a': 1, 'b': 2, 'c': 3 };

// keep the key and modify the value
let obj = objectMap(myObject, val => val * 2);
// obj = { a: 2, b: 4, c: 6 }


// modify both key and value
obj = objectMap(myObject,
    val => val * 2 + '',
    key => (key + key).toUpperCase());
// obj = { AA: '2', BB: '4', CC: '6' }

Code (TypeScript):

interface Dictionary<T> {
    [key: string]: T;
}

function objectMap<TValue, TResult>(
    obj: Dictionary<TValue>,
    valSelector: (val: TValue, obj: Dictionary<TValue>) => TResult,
    keySelector?: (key: string, obj: Dictionary<TValue>) => string,
    ctx?: Dictionary<TValue>
) {
    const ret = {} as Dictionary<TResult>;
    for (const key of Object.keys(obj)) {
        const retKey = keySelector
            ? keySelector.call(ctx || null, key, obj)
            : key;
        const retVal = valSelector.call(ctx || null, obj[key], obj);
        ret[retKey] = retVal;
    }
    return ret;
}

If you are not using TypeScript then copy the above code in TypeScript Playground to get the JavaScript code.

Also, the reason I put keySelector after valSelector in the parameter list, is because it is optional.

* Some credit go to alexander-mills' answer.


const mapObject = (targetObject, callbackFn) => {
    if (!targetObject) return targetObject;
    if (Array.isArray(targetObject)){
        return targetObject.map((v)=>mapObject(v, callbackFn))
    }
    return Object.entries(targetObject).reduce((acc,[key, value]) => {
        const res = callbackFn(key, value);
        if (!Array.isArray(res) && typeof res ==='object'){
            return {...acc, [key]: mapObject(res, callbackFn)}
        }
        if (Array.isArray(res)){
            return {...acc, [key]: res.map((v)=>mapObject(v, callbackFn))}
        }
        return {...acc, [key]: res};
    },{})
};
const mapped = mapObject(a,(key,value)=> {
    if (!Array.isArray(value) && key === 'a') return ;
    if (!Array.isArray(value) && key === 'e') return [];
    if (!Array.isArray(value) && key === 'g') return value * value;
    return value;
});
console.log(JSON.stringify(mapped)); 
// {"b":2,"c":[{"d":2,"e":[],"f":[{"g":4}]}]}

This function goes recursively through the object and arrays of objects. Attributes can be deleted if returned undefined


You can convert an object to array simply by using the following:

You can convert the object values to an array:

myObject = { 'a': 1, 'b': 2, 'c': 3 };

let valuesArray = Object.values(myObject);

console.log(valuesArray);

You can convert the object keys to an array:

myObject = { 'a': 1, 'b': 2, 'c': 3 };

let keysArray = Object.keys(myObject);

console.log(keysArray);

Now you can do perform normal array operations, including the 'map' function