Dynamically access object property using variable

Dynamically access object property using variable

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!”

Solutions/Answers:

Solution 1:

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

Solution 2:

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'})

Solution 3:

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

Solution 4:

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

Solution 5:

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'

Solution 6:

You can do it like this using Lodash get

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