Year 2017 answer: Object.assign()

Object.assign(dest, src1, src2, ...) merges objects.

It overwrites dest with properties and values of (however many) source objects, then returns dest.

The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object.

Live example

var obj = {key1: "value1", key2: "value2"};

Object.assign(obj, {key3: "value3"});



document.body.innerHTML = JSON.stringify(obj);

Year 2018 answer: object spread operator {...}

obj = {...obj, ...pair};

From MDN:

It copies own enumerable properties from a provided object onto a new object.

Shallow-cloning (excluding prototype) or merging of objects is now possible using a shorter syntax than Object.assign().

Note that Object.assign() triggers setters whereas spread syntax doesn’t.

Live example

It works in current Chrome and current Firefox. They say it doesn’t work in current Edge.

var obj = {key1: "value1", key2: "value2"};

var pair = {key3: "value3"};

obj = {...obj, ...pair};



document.body.innerHTML = JSON.stringify(obj);

Year 2019 answer

Object assignment operator +=:

obj += {key3: "value3"};

Oops... I got carried away. Smuggling information from the future is illegal. Duly obscured!

I have grown fond of the LoDash / Underscore when writing larger projects.

Adding by obj['key'] or obj.key are all solid pure JavaScript answers. However both of LoDash and Underscore libraries do provide many additional convenient functions when working with Objects and Arrays in general.

.push() is for Arrays, not for objects.

Depending what you are looking for, there are two specific functions that may be nice to utilize and give functionality similar to the the feel of arr.push(). For more info check the docs, they have some great examples there.

_.merge (Lodash only)

The second object will overwrite or add to the base object.
undefined values are not copied.

var obj = {key1: "value1", key2: "value2"};

var obj2 = {key2:"value4", key3: "value3", key4: undefined};

_.merge(obj, obj2);

console.log(obj);

// → {key1: "value1", key2: "value4", key3: "value3"} 

_.extend / _.assign

The second object will overwrite or add to the base object.
undefined will be copied.

var obj = {key1: "value1", key2: "value2"};

var obj2 = {key2:"value4", key3: "value3", key4: undefined};

_.extend(obj, obj2);

console.log(obj);

// → {key1: "value1", key2: "value4", key3: "value3", key4: undefined}

_.defaults

The second object contains defaults that will be added to base object if they don't exist.
undefined values will be copied if key already exists.

var obj = {key3: "value3", key5: "value5"};

var obj2 = {key1: "value1", key2:"value2", key3: "valueDefault", key4: "valueDefault", key5: undefined};

_.defaults(obj, obj2);

console.log(obj);

// → {key3: "value3", key5: "value5", key1: "value1", key2: "value2", key4: "valueDefault"}

$.extend

In addition, it may be worthwhile mentioning jQuery.extend, it functions similar to _.merge and may be a better option if you already are using jQuery.

The second object will overwrite or add to the base object.
undefined values are not copied.

var obj = {key1: "value1", key2: "value2"};

var obj2 = {key2:"value4", key3: "value3", key4: undefined};

$.extend(obj, obj2); 

console.log(obj);

// → {key1: "value1", key2: "value4", key3: "value3"}

Object.assign()

It may be worth mentioning the ES6/ ES2015 Object.assign, it functions similar to _.merge and may be the best option if you already are using an ES6/ES2015 polyfill like Babel if you want to polyfill yourself.

The second object will overwrite or add to the base object.
undefined will be copied.

var obj = {key1: "value1", key2: "value2"};

var obj2 = {key2:"value4", key3: "value3", key4: undefined};

Object.assign(obj, obj2); 

console.log(obj);

// → {key1: "value1", key2: "value4", key3: "value3", key4: undefined}

You could use either of these (provided key3 is the acutal key you want to use)

arr[ 'key3' ] = value3;

or

arr.key3 = value3;

If key3 is a variable, then you should do:

var key3 = 'a_key';

var value3 = 3;

arr[ key3 ] = value3;

After this, requesting arr.a_key would return the value of value3, a literal 3.

arr.key3 = value3;

because your arr is not really an array... It's a prototype object. The real array would be:

var arr = [{key1: value1}, {key2: value2}];

but it's still not right. It should actually be:

var arr = [{key: key1, value: value1}, {key: key2, value: value2}];

var employees = ; 

employees.push({id:100,name:'Yashwant',age:30});

employees.push({id:200,name:'Mahesh',age:35});

I know there is already an accepted answer for this but I thought I'd document my idea somewhere. Please [people] feel free to poke holes in this idea, as I'm not sure if it is the best solution... but I just put this together a few minutes ago:

Object.prototype.push = function( key, value ){

   this[ key ] = value;

   return this;

}

You would utilize it in this way:

var obj = {key1: value1, key2: value2};

obj.push( "key3", "value3" );

Since, the prototype function is returning this you can continue to chain .push's to the end of your obj variable: obj.push(...).push(...).push(...);

Another feature is that you can pass an array or another object as the value in the push function arguments. See my fiddle for a working example: http://jsfiddle.net/7tEme/

You can create a class with the answer of @Ionuț G. Stan

function obj(){

    obj=new Object();

    this.add=function(key,value){

        obj[""+key+""]=value;

    }

    this.obj=obj

}

Creating a new object with the last class:

my_obj=new obj();

my_obj.add('key1', 'value1');

my_obj.add('key2', 'value2');

my_obj.add('key3','value3');

Printing the object

console.log(my_obj.obj) // Return {key1: "value1", key2: "value2", key3: "value3"} 

Printing a Key

console.log(my_obj.obj["key3"]) //Return value3

I'm newbie in javascript, comments are welcome. Works for me.

Your example shows an Object, not an Array. In that case, the preferred way to add a field to an Object is to just assign to it, like so:

arr.key3 = value3;

Two most used ways already mentioned in most answers

obj.key3 = "value3";



obj["key3"] = "value3";

One more way to define a property is using Object.defineProperty()

Object.defineProperty(obj, 'key3', {

  value: "value3",       // undefined by default

  enumerable: true,      // false by default

  configurable: true,    // false by default

  writable: true         // false by default

});

This method is useful when you want to have more control while defining property.
Property defined can be set as enumerable, configurable and writable by user.

In case you have multiple anonymous Object literals inside an Object and want to add another Object containing key/value pairs, do this:

Firebug' the Object:

console.log(Comicbook);

returns:

[Object { name="Spiderman", value="11"}, Object { name="Marsipulami",
value="18"}, Object { name="Garfield", value="2"}]

Code:

if (typeof Comicbook[3]=='undefined') {

    private_formArray[3] = new Object();

    private_formArray[3]["name"] = "Peanuts";

    private_formArray[3]["value"] = "12";

}

will add Object {name="Peanuts", value="12"} to the Comicbook Object

Either obj['key3'] = value3 or obj.key3 = value3 will add the new pair to the obj.

However, I know jQuery was not mentioned, but if you're using it, you can add the object through $.extend(obj,{key3: 'value3'}). E.g.:

var obj = {key1: 'value1', key2: 'value2'};

$('#ini').append(JSON.stringify(obj));



$.extend(obj,{key3: 'value3'});



$('#ext').append(JSON.stringify(obj));

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<p id="ini">Initial: </p>

<p id="ext">Extended: </p>

jQuery.extend(target[,object1][,objectN]) merges the contents of two or more objects together into the first object.

And it also allows recursive adds/modifications with $.extend(true,object1,object2);:

var object1 = {

  apple: 0,

  banana: { weight: 52, price: 100 },

  cherry: 97

};

var object2 = {

  banana: { price: 200 },

  durian: 100

};

$("#ini").append(JSON.stringify(object1));    



$.extend( true, object1, object2 );

 

$("#ext").append(JSON.stringify(object1));

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<p id="ini">Initial: </p>

<p id="ext">Extended: </p>

You can either add it this way:

arr['key3'] = value3;

or this way:

arr.key3 = value3;

The answers suggesting keying into the object with the variable key3 would only work if the value of key3 was 'key3'.

According to Property Accessors defined in ECMA-262(http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf, P67), there are two ways you can do to add properties to a exists object. All these two way, the Javascript engine will treat them the same.

The first way is to use dot notation:

obj.key3 = value3;

But this way, you should use a IdentifierName after dot notation.

The second way is to use bracket notation:

obj["key3"] = value3;

and another form:

var key3 = "key3";

obj[key3] = value3;

This way, you could use a Expression (include IdentifierName) in the bracket notation.

var arrOfObj = [{name: 'eve'},{name:'john'},{name:'jane'}];

    var injectObj = {isActive:true, timestamp:new Date()};



    // function to inject key values in all object of json array



    function injectKeyValueInArray (array, keyValues){

        return new Promise((resolve, reject) => {

            if (!array.length)

                return resolve(array);



            array.forEach((object) => {

                for (let key in keyValues) {

                    object[key] = keyValues[key]

                }

            });

            resolve(array);

        })

    };



//call function to inject json key value in all array object

    injectKeyValueInArray(arrOfObj,injectObj).then((newArrOfObj)=>{

        console.log(newArrOfObj);

    });

Output like this:-

[ { name: 'eve',

    isActive: true,

    timestamp: 2017-12-16T16:03:53.083Z },

  { name: 'john',

    isActive: true,

    timestamp: 2017-12-16T16:03:53.083Z },

  { name: 'jane',

    isActive: true,

    timestamp: 2017-12-16T16:03:53.083Z } ]

A short and elegant way in next Javascript specification (candidate stage 3) is:

obj = { ... obj, ... { key3 : value3 } }

A deeper discussion can be found in Object spread vs Object.assign and on Dr. Axel Rauschmayers site.

It works already in node.js since release 8.6.0.

Vivaldi, Chrome, Opera, and Firefox in up to date releases know this feature also, but Mirosoft don't until today, neither in Internet Explorer nor in Edge.

Simply adding properties:

And we want to add prop2 : 2 to this object, these are the most convenient options:

1. Dot operator: object.prop2 = 2;
   


2. square brackets: object['prop2'] = 2;
   

So which one do we use then?

The dot operator is more clean syntax and should be used as a default (imo). However, the dot operator is not capable of adding dynamic keys to an object, which can be very useful in some cases. Here is an example:

const obj = {

  prop1: 1

}



const key = Math.random() > 0.5 ? 'key1' : 'key2';



obj[key] = 'this value has a dynamic key';



console.log(obj);

Merging objects:

When we want to merge the properties of 2 objects these are the most convenient options:

1. 
   Object.assign(), takes a target object as an argument, and one or more source objects and will merge them together. For example:

const object1 = {

  a: 1,

  b: 2,

};



const object2 = Object.assign({

  c: 3,

  d: 4

}, object1);



console.log(object2);

2. Object spread operator ...
   

const obj = {

  prop1: 1,

  prop2: 2

}



const newObj = {

  ...obj,

  prop3: 3,

  prop4: 4

}



console.log(newObj);

Which one do we use?

• The spread syntax is less verbose and has should be used as a default imo. Don't forgot to transpile this syntax to syntax which is supported by all browsers because it is relatively new.


• 
  Object.assign() is more dynamic because we have access to all objects which are passed in as arguments and can manipulate them before they get assigned to the new Object.

We can do this in this way too.

var myMap = new Map();

myMap.set(0, 'my value1');

myMap.set(1, 'my value2');

 for (var [key, value] of myMap) {

  console.log(key + ' = ' + value);

 }

Since its a question of the past but the problem of present. Would suggest one more solution: Just pass the key and values to the function and you will get a map object.

var map = {};

function addValueToMap(key, value) {

map[key] = map[key] || ;

map[key].push(value);

}

supported by most of browsers, and it checks if object key available or not you want to add, if available it overides existing key value and it not available it add key with value

example 1

let my_object = {};



// now i want to add something in it  



my_object.red = "this is red color";



// { red : "this is red color"}

example 2

let my_object = { inside_object : { car : "maruti" }}



// now i want to add something inside object of my object 



my_object.inside_object.plane = "JetKing";



// { inside_object : { car : "maruti" , plane : "JetKing"} }

example 3

let my_object = { inside_object : { name : "abhishek" }}



// now i want to add something inside object with new keys birth , gender 



my_object.inside_object.birth = "8 Aug";

my_object.inside_object.gender = "Male";





    // { inside_object : 

//             { name : "abhishek",

//               birth : "8 Aug",

//               gender : "Male" 

//            }   

//       }

In order to prepend a key-value pair to an object so the for in works with that element first do this:

    var nwrow = {'newkey': 'value' };

    for(var column in row){

        nwrow[column] = row[column];

    }

    row = nwrow;

Best way to achieve same is stated below:

function getKey(key) {

  return `${key}`;

}



var obj = {key1: "value1", key2: "value2", [getKey('key3')]: "value3"};



//console.log(obj);