Object.fromEntries()

Object.fromEntries() is a static method that transforms a list of key-value pairs into an object. It takes an iterable object, such as an array, and returns a new object with properties corresponding to the key-value pairs. The iterable parameter must contain elements that are iterable themselves and have exactly two elements in them, representing key-value pairs. Object.fromEntries() was introduced in ECMAScript 2019 and may not be supported in all browsers.

Syntax

Object.fromEntries(iterable);

Parameter

• iterable: An iterable object, such as an array, that contains key-value pairs.

Return Value

A new object with properties corresponding to the key-value pairs.

Examples

Converting an array of key-value pairs to an object

const entries = [['a', 1], ['b', 2], ['c', 3]];
const obj = Object.fromEntries(entries);
console.log(obj); // { a: 1, b: 2, c: 3 }

Converting a Map object to an object

const map = new Map([['a', 1], ['b', 2], ['c', 3]]);
const obj = Object.fromEntries(map);
console.log(obj); // { a: 1, b: 2, c: 3 }

Converting an array of objects with key-value pairs to an object

const arr = [{ key: 'a', value: 1 }, { key: 'b', value: 2 }, { key: 'c', value: 3 }];
const obj = Object.fromEntries(arr.map(({ key, value }) => [key, value]));
console.log(obj); // { a: 1, b: 2, c: 3 }

Notes

• If the iterable contains duplicate keys, the last value will overwrite the previous values.

• If an object with a prototype is passed to Object.fromEntries(), the resulting object will have the same prototype as the original object.

• If the iterable contains non-string keys, they will be converted to strings before being used as property names.

• This method is the inverse of Object.entries(), meaning it creates an object from an array of key-value pairs.

• The input array must be an iterable object, such as an array or a Map object.

• If any element of the input array is not an array, or if the sub-array does not have exactly two elements, an error will be thrown.

• The resulting object will have its own prototype, and will not inherit properties from Object.prototype.

• The method is only available in ES2019 and later, so it may not be available in older browsers or environments.

Exercises

1. Convert an object to an array of key-value pairs using Object.entries() and then convert it back to an object using Object.fromEntries().

2. Use Object.fromEntries() to create an object from a URLSearchParams object.

3. Use Object.fromEntries() to create a new object from an existing object with the values of all the properties multiplied by 2.

const obj = { a: 1, b: 2, c: 3 };
const arr = Object.entries(obj);
const newObj = Object.fromEntries(arr);
console.log(newObj); // { a: 1, b: 2, c: 3 }

const params = new URLSearchParams('key1=value1&key2=value2');
const obj = Object.fromEntries(params);
console.log(obj); // { key1: 'value1', key2: 'value2' }

const obj = { a: 1, b: 2, c: 3 };
const entries = Object.entries(obj).map(([key, value]) => [key, value * 2]);
const newObj = Object.fromEntries(entries);
console.log(newObj); // { a: 2, b: 4, c: 6 }