Lodash fromPairs: Convert Arrays to Objects

The Lodash library provides a wide range of utility functions to manipulate collections, with _.fromPairs being specifically designed for structural transformation. This article explains the exact data structure that _.fromPairs converts into a JavaScript object, how the function processes elements, and how it compares to native JavaScript alternatives.

The Input Data Structure

The _.fromPairs function converts a two-dimensional array of key-value pairs—frequently referred to as an array of pairs, entries, or two-element tuples—into a standard JavaScript object.

In this structure:

• The outer container must be an Array.
• Each item inside the outer array must be an array where the first element represents the object property key and the second element represents the corresponding value.

Structure Example

[
  ['key1', 'value1'],
  ['key2', 'value2'],
  ['key3', 'value3']
]

How It Works in Practice

When _.fromPairs processes this nested array, it iterates through the outer list, extracts the first two elements of each sub-array, and assigns them as property-value pairs on a newly created object.

const _ = require('lodash');

const entries = [
  ['id', 101],
  ['username', 'johndoe'],
  ['isActive', true]
];

const userObject = _.fromPairs(entries);

console.log(userObject);
// Output: { id: 101, username: 'johndoe', isActive: true }

Handling Duplicates and Irregular Inputs

• Duplicate Keys: If the input array contains sub-arrays with the same key, subsequent occurrences will overwrite earlier ones, matching standard JavaScript object behavior.
• Non-String Keys: Any non-string primitive used as a key in the first position is automatically coerced to a string or Symbol according to standard JavaScript object key rules.
• Length of Sub-arrays: If an inner array contains more than two items, elements beyond the second index are ignored. If an inner array contains only one item, the key is set and its value becomes undefined.

Native Alternative

In modern JavaScript (ES2019 and later), this transformation can be achieved natively using Object.fromEntries(), which accepts the same nested array data structure without requiring an external library dependency.