How do I create a dynamic key to be added to a JavaScript object variable

javascript

prompt-engineering

functions

callbacks

byNikita Barsukov·Dec 15, 2024

To add a dynamic key to a JavaScript object, use the bracket notation as follows:

let key = 'userID';
let value = 123;
let obj = {[key]: value};

The obj now boasts a property userID with a value of 123, derived from the key and value variables.

Deeper dive into dynamic keys

JavaScript objects are an integral part of most programming tasks. For added flexibility, you can use dynamic keys to build objects based on runtime values. Let's investigate this further.

ES6 computed property names

let propertyName = 'status';
let object = {
  [propertyName]: 'active',  // ES6 magic right here!
};

ES6 inaugurates computed property names which evaluates expressions inside the bracket syntax of an object literal.

Arrays vs object properties

The bracket notation isn't reserved only for objects; arrays can employ it too. But watch your step: assigning a numeric key to an array may inflate its length. It's witchcraft, but not the Hogwarts kind:

let arr = [];
arr['5'] = 'value';  // Do you feel the dark side, Luke?
console.log(arr.length);  // Outputs 6

This also results in a newcomer to the JSON serialization party: only elements indexed numerically get invited to this jam.

Using loops for sequential keys

To create a sequence of dynamic keys, loops come very handy:

let baseKey = 'item';
for (let i = 0; i < 3; i++) {
  object[baseKey + i] = `value${i}`;  // Not just items, but item0, item1, item2
}

In this fashion, you can weave string and numeric values to spawn sequential keys, much like ducks in a row.

Shaking hands with `for...in`

For the adventurous, a for...in loop might look attractive. However, be mindful of object inheritance traps. Unlike Indiana Jones, you might not have a whip to avoid them:

for (let prop in object) {
  if (object.hasOwnProperty(prop)) {
    // Always knock before entering. Politeness 101.
  }
}

To keep things family-friendly, filter out properties from the prototype chain with hasOwnProperty.

In a world where arrays aren't arrays

In a land far, far away, associative arrays feature indexes with various values. But no such luck in JavaScript; here, arrays are zero-based, indexed with integers, and stick to their roots:

let arr = [];
arr['keyName'] = 'value';  // Wait, is that an array or an object?

Nevertheless, adding string keys to your arrays doesn't transform them into associative arrays, but instills more object-like behavior instead.

Enhancing objects with `Object.defineProperty`

Object.defineProperty, the Swiss knife of JavaScript, is a game-changer for customizing property attributes:

Object.defineProperty(object, 'newKey', {
  value: 'newValue',
  writable: true,
  enumerable: true,
  configurable: true
});

Suddenly, your property has different faces: writable, enumerable, and configurable. Fascinating, isn't it?

More than meets the `Map`

If you need keys that aren't necessarily strings, or order matters to you, you might find Map interesting:

let map = new Map();
map.set(anyKey, 'anyValue');

The Map object is like the cool kid that doesn't conform to the norm: it can use any value for a key and cares about the order of insertion too.

Jazzing up JSON data with dynamic keys

Working with JSON data is a breeze with dynamic keys, making it easy to transform the data based on application logic or server responses:

let jsonData = '{"id":1,"name":"Alice"}';
let dynamicKey = 'isActive';
let userObject = JSON.parse(jsonData);
userObject[dynamicKey] = true;  // Alice is now active. Go, Alice!