JavaScript Object: Access Variable Property by Name as String

JavaScript object: access variable property by name as string

You don't need a function for it - simply use the bracket notation:

var side = columns['right'];

This is equal to dot notation, var side = columns.right;, except the fact that right could also come from a variable, function return value, etc., when using bracket notation.

If you NEED a function for it, here it is:

function read_prop(obj, prop) {
return obj[prop];

To answer some of the comments below that aren't directly related to the original question, nested objects can be referenced through multiple brackets. If you have a nested object like so:

var foo = { a: 1, b: 2, c: {x: 999, y:998, z: 997}};

you can access property x of c as follows:

var cx = foo['c']['x']

If a property is undefined, an attempt to reference it will return undefined (not null or false):

foo['c']['q'] === null
// returns false

foo['c']['q'] === false
// returns false

foo['c']['q'] === undefined
// returns true

Dynamically access object property using variable

There are two ways to access properties of an object:

  • Dot notation:
  • Bracket notation: something['bar']

The value between the brackets can be any expression. Therefore, if the property name is stored in a variable, you have to use bracket notation:

var something = {
bar: 'foo'
var foo = 'bar';

// both x = something[foo] and something[foo] = x work as expected

How to get the property name of object and use that property name in javascript

The only error in your code is mapped_errors.{obj_key}.msg. You should be using [] to access the object's property value when you are using a variable to get the key name, as mapped_errors[obj_key].

var mapped_errors = { _id:

{ location: 'body',

param: '_id',

value: 123,

msg: '_id is required and must be a string' } };

let obj_key = Object.keys(mapped_errors);


mapped_errors = { _name:

{ location: 'body',

param: '_name',

value: 123,

msg: '_name is required and must be a string' } }

obj_key = Object.keys(mapped_errors);


How to dynamically access object property in TypeScript

"OK" is a string, and str is implicitly taking the type string in your code.

When you try to access an object's property, you need to use a type keyof. TypeScript then knows you are not assigning a random string; you are assigning strings compatible with the properties (keys) for the object.

Also, since status is a variable, not a type, you need to extract its type with typeof.


let str = "OK" as keyof typeof status;
status[str]; // 200

or more cleanly:

type StatusKey = keyof typeof status;
let str: StatusKey = "OK";
status[str]; // 200

// and to answer the question about reversal
status[status.OK as StatusKey]; // OK


Dynamically access object properties using a variable

Make use of brackets notation for accessing dynamic keys

var my_object = {

object1: {

key: 'value',

key2: 'value2'


object2: {

key: 'othervalue',

key2: 'another'



function doSomething(obj_key) {

// add a new property to the object

my_object[obj_key].new_prop = 'this_new_prop'; // using bracket notation here




Getting a Custom Objects properties by string var

Simply use myObject['thing'].

Related Topics

Leave a reply