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?
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();
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 .