How can I add a key/value pair to a JavaScript object?

How can I add a key/value pair to a JavaScript object?

Here is my object literal:
var obj = {key1: value1, key2: value2};

How can I add {key3: value3} to the object?

Solutions/Answers:

Solution 1:

There are two ways to add new properties to an object:

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

Using dot notation:

obj.key3 = "value3";

Using square bracket notation:

obj["key3"] = "value3";

The first form is used when you know the name of the property. The second form is used when the name of the property is dynamically determined. Like in this example:

var getProperty = function (propertyName) {
    return obj[propertyName];
};

getProperty("key1");
getProperty("key2");
getProperty("key3");

A real JavaScript array can be constructed using either:

The Array literal notation:

var arr = [];

The Array constructor notation:

var arr = new Array();

Solution 2:

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"};

Solution 3:

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}

Solution 4:

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.

Solution 5:

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}];

Solution 6:

var employees = []; 
employees.push({id:100,name:'Yashwant',age:30});
employees.push({id:200,name:'Mahesh',age:35});