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I'm trying to access a property of an object using a dynamic name. Is this possible?

const something = { bar: "Foobar!" };
const foo = 'bar';
something.foo; // The idea is to access something.bar, getting "Foobar!"

I'm trying to access a property of an object using a dynamic name. Is this possible?

const something = { bar: "Foobar!" };
const foo = 'bar';
something.foo; // The idea is to access something.bar, getting "Foobar!"

• javascript
• object
• properties

Share Improve this question Follow edited Nov 16, 2022 at 18:53 dumbass 27.2k44 gold badges3636 silver badges7373 bronze badges asked Nov 22, 2010 at 11:23 RichWRichW 10.9k77 gold badges2828 silver badges3333 bronze badges 1

• 5 See also property access: dot notation vs. brackets? and How do I add a property to an object using a variable as the name? – Bergi Commented Nov 18, 2014 at 6:11

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22 Answers 22

Sorted by: Reset to default 1303

There are two ways to access properties of an object:

• Dot notation: something.bar
• 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
console.log(something[foo]);
console.log(something.bar)

This is my solution:

function resolve(path, obj) {
    return path.split('.').reduce(function(prev, curr) {
        return prev ? prev[curr] : null
    }, obj || self)
}

Usage examples:

resolve("document.body.style.width")
// or
resolve("style.width", document.body)
// or even use array indexes
// (someObject has been defined in the question)
resolve("part.0.size", someObject) 
// returns null when intermediate properties are not defined:
resolve('properties.that.do.not.exist', {hello:'world'})

In javascript we can access with:

• dot notation - foo.bar
• square brackets - foo[someVar] or foo["string"]

But only second case allows to access properties dynamically:

var foo = { pName1 : 1, pName2 : [1, {foo : bar }, 3] , ...}

var name = "pName"
var num  = 1;

foo[name + num]; // 1

// -- 

var a = 2;
var b = 1;
var c = "foo";

foo[name + a][b][c]; // bar

Following is an ES6 example of how you can access the property of an object using a property name that has been dynamically generated by concatenating two strings.

var suffix = " name";

var person = {
    ["first" + suffix]: "Nicholas",
    ["last" + suffix]: "Zakas"
};

console.log(person["first name"]);      // "Nicholas"
console.log(person["last name"]);       // "Zakas"

This is called computed property names

You can achieve this in quite a few different ways.

let foo = {
    bar: 'Hello World'
};

foo.bar;
foo['bar'];

The bracket notation is specially powerful as it let's you access a property based on a variable:

let foo = {
    bar: 'Hello World'
};

let prop = 'bar';

foo[prop];

This can be extended to looping over every property of an object. This can be seem redundant due to newer JavaScript constructs such as for ... of ..., but helps illustrate a use case:

let foo = {
    bar: 'Hello World',
    baz: 'How are you doing?',
    last: 'Quite alright'
};

for (let prop in foo.getOwnPropertyNames()) {
    console.log(foo[prop]);
}

Both dot and bracket notation also work as expected for nested objects:

let foo = {
    bar: {
        baz: 'Hello World'
    }
};

foo.bar.baz;
foo['bar']['baz'];
foo.bar['baz'];
foo['bar'].baz;

Object destructuring

We could also consider object destructuring as a means to access a property in an object, but as follows:

let foo = {
    bar: 'Hello World',
    baz: 'How are you doing?',
    last: 'Quite alright'
};

let prop = 'last';
let { bar, baz, [prop]: customName } = foo;

// bar = 'Hello World'
// baz = 'How are you doing?'
// customName = 'Quite alright'

You can do it like this using Lodash get

_.get(object, 'a[0].b.c');

UPDATED

Accessing root properties in an object is easily achieved with obj[variable], but getting nested complicates things. Not to write already written code I suggest to use lodash.get.

Example

// Accessing root property
var rootProp = 'rootPropert';
_.get(object, rootProp, defaultValue);

// Accessing nested property
var listOfNestedProperties = [var1, var2];
_.get(object, listOfNestedProperties);

Lodash get can be used in different ways, the documentation lodash.get

To access a property dynamically, simply use square brackets [] as follows:

const something = { bar: "Foobar!" };
const userInput = 'bar';
console.log(something[userInput])

The problem

There's a major gotchya in that solution! (I'm surprised other answers have not brought this up yet). Often you only want to access properties that you've put onto that object yourself, you don't want to grab inherited properties.

Here's an illustration of this issue. Here we have an innocent-looking program, but it has a subtle bug - can you spot it?

const agesOfUsers = { sam: 16, sally: 22 }
const username = prompt('Enter a username:')
if (agesOfUsers[username] !== undefined) {
  console.log(`${username} is ${agesOfUsers[username]} years old`)
} else {
  console.log(`${username} is not found`)
}

When prompted for a username, if you supply "toString" as a username, it'll give you the following message: "toString is function toString() { [native code] } years old". The issue is that agesOfUsers is an object, and as such, automatically inherits certain properties like .toString() from the base Object class. You can look here for a full list of properties that all objects inherit.

Solutions

1. Use a Map data structure instead. The stored contents of a map don't suffer from prototype issues, so they provide a clean solution to this problem.

const agesOfUsers = new Map()
agesOfUsers.set('sam', 16)
agesOfUsers.set('sally', 2)
console.log(agesOfUsers.get('sam')) // 16

  

2. Use an object with a null prototype, instead of the default prototype. You can use Object.create(null) to create such an object. This sort of object does not suffer from these prototype issues, because you've explicitly created it in a way that it does not inherit anything.

const agesOfUsers = Object.create(null)
agesOfUsers.sam = 16
agesOfUsers.sally = 22;
console.log(agesOfUsers['sam']) // 16
console.log(agesOfUsers['toString']) // undefined - toString was not inherited

  

3. You can use Object.hasOwn(yourObj, attrName) to first check if the dynamic key you wish to access is directly on the object and not inherited (learn more here). This is a relatively newer feature, so check the compatibility tables before dropping it into your code. Before Object.hasOwn(yourObj, attrName) came around, you would achieve this same effect via Object.prototype.hasOwnProperty.call(yourObj, attrName). Sometimes, you might see code using yourObj.hasOwnProperty(attrName) too, which sometimes works but it has some pitfalls that you can read about here.

// Try entering the property name "toString",
// you'll see it gets handled correctly.
const user = { name: 'sam', age: 16 }
const propName = prompt('Enter a property name:')
if (Object.hasOwn(user, propName)) {
  console.log(`${propName} = ${user[propName]}`)
} else {
  console.log(`${propName} is not found`)
}

4. If you know the key you're trying to use will never be the name of an inherited property (e.g. maybe they're numbers, or they all have the same prefix, etc), you can choose to use the original solution.

I came across a case where I thought I wanted to pass the "address" of an object property as data to another function and populate the object (with AJAX), do lookup from address array, and display in that other function. I couldn't use dot notation without doing string acrobatics so I thought an array might be nice to pass instead. I ended-up doing something different anyway, but seemed related to this post.

Here's a sample of a language file object like the one I wanted data from:

const locs = {
  "audioPlayer": {
    "controls": {
      "start": "start",
      "stop": "stop"
    },
    "heading": "Use controls to start and stop audio."
  }
}

I wanted to be able to pass an array such as: ["audioPlayer", "controls", "stop"] to access the language text, "stop" in this case.

I created this little function that looks-up the "least specific" (first) address parameter, and reassigns the returned object to itself. Then it is ready to look-up the next-most-specific address parameter if one exists.

function getText(selectionArray, obj) {
  selectionArray.forEach(key => {
    obj = obj[key];
  });
  return obj;
}

usage:

/* returns 'stop' */
console.log(getText(["audioPlayer", "controls", "stop"], locs)); 

/* returns 'use controls to start and stop audio.' */
console.log(getText(["audioPlayer", "heading"], locs)); 

ES5 // Check Deeply Nested Variables

This simple piece of code can check for deeply nested variable / value existence without having to check each variable along the way...

var getValue = function( s, context ){
    return Function.call( context || null, 'return ' + s )();
}

Ex. - a deeply nested array of objects:

a = [ 
    {
      b : [
          {
             a : 1,
             b : [
                 {
                    c : 1,
                    d : 2   // we want to check for this
                 }
             ]
           }
      ]
    } 
]

Instead of :

if(a && a[0] && a[0].b && a[0].b[0] && a[0].b[0].b && a[0].b[0].b[0] && a[0].b[0].b[0].d && a[0].b[0].b[0].d == 2 )  // true

We can now :

if( getValue('a[0].b[0].b[0].d') == 2 ) // true

Cheers!

Others have already mentioned 'dot' and 'square' syntaxes so I want to cover accessing functions and sending parameters in a similar fashion.

Code jsfiddle

var obj = {method:function(p1,p2,p3){console.log("method:",arguments)}}

var str = "method('p1', 'p2', 'p3');"

var match = str.match(/^\s*(\S+)\((.*)\);\s*$/);

var func = match[1]
var parameters = match[2].split(',');
for(var i = 0; i < parameters.length; ++i) {
  // clean up param begninning
    parameters[i] = parameters[i].replace(/^\s*['"]?/,'');
  // clean up param end
  parameters[i] = parameters[i].replace(/['"]?\s*$/,'');
}

obj[func](parameters); // sends parameters as array
obj[func].apply(this, parameters); // sends parameters as individual values

demo object example

let obj = {
    name: {
        first_name: "Bugs",
        last_name: "Founder",
        role: "Programmer"
    }
}

dotted string key for getting the value of

let key = "name.first_name"

Function

const getValueByDottedKeys = (obj, strKey)=>{
    let keys = strKey.split(".")
    let value = obj[keys[0]];   
    for(let i=1;i<keys.length;i++){
        value = value[keys[i]]
    }
    return value
}

Calling getValueByDottedKeys function

value = getValueByDottedKeys(obj, key)
console.log(value)

output

Bugs

const getValueByDottedKeys = (obj, strKey)=>{
    let keys = strKey.split(".")
    let value = obj[keys[0]];   
    for(let i=1;i<keys.length;i++){
        value = value[keys[i]]
    }
    return value
}

let obj = {
    name: {
        first_name: "Bugs",
        last_name: "Founder",
        role: "Programmer"
    }
}

let key = "name.first_name"

value = getValueByDottedKeys(obj, key)
console.log(value)

I asked a question that kinda duplicated on this topic a while back, and after excessive research, and seeing a lot of information missing that should be here, I feel I have something valuable to add to this older post.

• Firstly I want to address that there are several ways to obtain the value of a property and store it in a dynamic Variable. The first most popular, and easiest way IMHO would be:

let properyValue = element.style['enter-a-property'];

however I rarely go this route because it doesn't work on property values assigned via style-sheets. To give you an example, I'll demonstrate with a bit of pseudo code.

 let elem = document.getElementById('someDiv');
 let cssProp = elem.style['width'];

Using the code example above; if the width property of the div element that was stored in the 'elem' variable was styled in a CSS style-sheet, and not styled inside of its HTML tag, you are without a doubt going to get a return value of undefined stored inside of the cssProp variable. The undefined value occurs because in-order to get the correct value, the code written inside a CSS Style-Sheet needs to be computed in-order to get the value, therefore; you must use a method that will compute the value of the property who's value lies within the style-sheet.

• Henceforth the getComputedStyle() method!

function getCssProp(){
  let ele = document.getElementById("test");
  let cssProp = window.getComputedStyle(ele,null).getPropertyValue("width");
}

W3Schools getComputedValue Doc This gives a good example, and lets you play with it, however, this link Mozilla CSS getComputedValue doc talks about the getComputedValue function in detail, and should be read by any aspiring developer who isn't totally clear on this subject.

• As a side note, the getComputedValue method only gets, it does not set. This, obviously is a major downside, however there is a method that gets from CSS style-sheets, as well as sets values, though it is not standard Javascript. The JQuery method...

$(selector).css(property,value)

...does get, and does set. It is what I use, the only downside is you got to know JQuery, but this is honestly one of the very many good reasons that every Javascript Developer should learn JQuery, it just makes life easy, and offers methods, like this one, which is not available with standard Javascript. Hope this helps someone!!!

For anyone looking to set the value of a nested variable, here is how to do it:

const _ = require('lodash'); //import lodash module

var object = { 'a': [{ 'b': { 'c': 3 } }] };

_.set(object, 'a[0].b.c', 4);
console.log(object.a[0].b.c);
// => 4

Documentation: https://lodash.com/docs/4.17.15#set

Also, documentation if you want to get a value: https://lodash.com/docs/4.17.15#get

You can do dynamically access the property of an object using the bracket notation. This would look like this obj[yourKey] however JavaScript objects are really not designed to dynamically updated or read. They are intended to be defined on initialisation.

In case you want to dynamically assign and access key value pairs you should use a map instead.

const yourKey = 'yourKey';

// initialise it with the value
const map1 = new Map([
  ['yourKey', 'yourValue']
]);

// initialise empty then dynamically assign
const map2 = new Map();
map2.set(yourKey, 'yourValue');

console.log(map1.get(yourKey));
console.log(map2.get(yourKey));

I bumped into the same problem, but the lodash module is limited when handling nested properties. I wrote a more general solution following the idea of a recursive descendent parser. This solution is available in the following Gist:

Recursive descent object dereferencing

Here is one way to access object property dynamically, Case is to access JSON Object based on localstorage data {preferredIndustry= foodItemsFeatured}

data.foodItemsFeatured to be accessed

let storageData = this.data; // JSON Response
let preferredData = localStorage.getItem('preferredIndustry'); //foodItemsFeatured

/* Dynamically access Object's property */
this.preferredFeaturedItems = this.data[preferredData as keyof typeof storageData];

If you want to avoid the ES6 {[prop]:val} method and be able to use it in an arrow function:

var obj = {a:1}, prop = "b", val = 2;
console.log(JSON.stringify(obj)); // {"a":1}
Object.defineProperty(obj, prop, {value:val, writable:true,enumerable:true,configurable:true});
console.log(JSON.stringify(obj)); // {"a":1,"b":2};

Arrow function:

var set = (obj, prop, val) => Object.defineProperty(obj, prop, {value:val, writable:true,enumerable:true,configurable:true});
set(obj, "c", 3);
console.log(JSON.stringify(obj));// {"a":1,"b":2, "c":3};

I use below solution. It seems easier to read than the reduce function.

It uses the bracket notation to get the property value. It supports getting property value of nested object by providing a dot ('.') separated key. If a property in the supplied key is not present in the object, undefined is returned. This is achieved by using the safe chaining operator "?.".

function getPropertyValue(object, key) {
  key.split(".").forEach(token => object = object?.[token]);
  return object;
}

// Examples
const o = { a: 1, b: { c: 2, d: 3 } };
console.log(getPropertyValue(o, "a"));
console.log(getPropertyValue(o, "b"));
console.log(getPropertyValue(o, "b.c"));

// Examples where properties does not exist
console.log(getPropertyValue(o, "d"));
console.log(getPropertyValue(o, "b.a"));
console.log(getPropertyValue(o, "d.c"));
console.log(getPropertyValue(o, "b.c.d"));

Finding Object by reference without, strings, Note make sure the object you pass in is cloned , i use cloneDeep from lodash for that

if object looks like

const obj = {data: ['an Object',{person: {name: {first:'nick', last:'gray'} }]

path looks like

const objectPath = ['data',1,'person',name','last']

then call below method and it will return the sub object by path given

const child = findObjectByPath(obj, objectPath)
alert( child) // alerts "last"


const findObjectByPath = (objectIn: any, path: any[]) => {
    let obj = objectIn
    for (let i = 0; i <= path.length - 1; i++) {
        const item = path[i]
        // keep going up to the next parent
        obj = obj[item] // this is by reference
    }
    return obj
}

You can use getter in Javascript

getter Docs

Check inside the Object whether the property in question exists, If it does not exist, take it from the window

const something = {
    get: (n) => this.n || something.n || window[n]
};

You should use JSON.parse, take a look at https://www.w3schools.com/js/js_json_parse.asp

const obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}')
console.log(obj.name)
console.log(obj.age)

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