Add a property to a JavaScript object using a variable as the name?


Add a property to a JavaScript object using a variable as the name?

I’m pulling items out of the DOM with jQuery and want to set a property on an object using the id of the DOM element.
const obj = {}

jQuery(itemsFromDom).each(function() {
const element = jQuery(this)
const name = element.attr(‘id’)
const value = element.attr(‘value’)

// Here is the problem = value

If itemsFromDom includes an element with an id of “myId”, I want obj to have a property named “myId”. The above gives me name.
How do I name a property of an object using a variable using JavaScript?


Solution 1:

You can use this equivalent syntax:

obj[name] = value

Solution 2:

With ECMAScript 2015 you can do it directly in object declaration using bracket notation:

var obj = {
  [key]: value

Where key can be any sort of expression (e.g. a variable) returning a value:

var obj = {
  ['hello']: 'World',
  [x + 2]: 42,
  [someObject.getId()]: someVar

Solution 3:

You can even make List of objects like this

var feeTypeList = [];
$('#feeTypeTable > tbody > tr').each(function (i, el) {
    var feeType = {};

    var $ID = $(this).find("input[id^=txtFeeType]").attr('id');

    feeType["feeTypeID"] = $('#ddlTerm').val();
    feeType["feeTypeName"] = $('#ddlProgram').val();
    feeType["feeTypeDescription"] = $('#ddlBatch').val();


Solution 4:

With lodash, you can create new object like this _.set:

obj = _.set({}, key, val);

Or you can set to existing object like this:

var existingObj = { a: 1 };
_.set(existingObj, 'a', 5); // existingObj will be: { a: 5 }

You should take care if you want to use dot (“.”) in your path, because lodash can set hierarchy, for example:

_.set({}, "a.b.c", "d"); // { "a": { "b": { "c": "d" } } }

Solution 5:

First we need to define key as variable and then we need to assign as key as object., for example

var data = {key:'dynamic_key',value:'dynamic_value'}
var key = data.key;
var obj = { [key]: data.value}

Solution 6:

There are two different notations to access object properties

  • Dot notation: myObj.prop1
  • Bracket notation: myObj[“prop1”]

Dot notation is fast and easy but you must use the actual property name explicitly. No substitution, variables, etc.

Bracket notation is open ended. It uses a string but you can produce the string using any legal js code. You may specify the string as literal (though in this case dot notation would read easier) or use a variable or calculate in some way.

So, these all set the myObj property named prop1 to the value Hello:

// quick easy-on-the-eye dot notation
myObj.prop1 = "Hello";

// brackets+literal
myObj["prop1"] = "Hello";

// using a variable
var x = "prop1"; 
myObj[x] = "Hello";                     

// calculate the accessor string in some weird way
var numList = [0,1,2];
myObj[ "prop" + numList[1] ] = "Hello";     


myObj.[xxxx] = "Hello";      // wrong: mixed notations, syntax fail
myObj[prop1] = "Hello";      // wrong: this expects a variable called prop1

tl;dnr: If you want to compute or reference the key you must use bracket notation. If you are using the key explicitly, then use dot notation for simple clear code.

Note: there are some other good and correct answers but I personally found them a bit brief coming from a low familiarity with JS on-the-fly quirkiness. This might be useful to some people.