Functional programming with lodash/fp

lodash/fp re-exports the same methods as the standard build, converted to be immutable, auto-curried, iteratee-first, and data-last. Use it when you want to build reusable, composable functions instead of one-off imperative calls.

var fp = require('lodash/fp');

How lodash/fp differs from the standard build

Auto-currying

Call an fp method with fewer arguments than it needs, and it returns a function waiting for the rest:

var double = fp.map(function (n) { return n * 2; });

double([1, 2, 3]);
// => [2, 4, 6]

var addOne = fp.add(1);
addOne(2);
// => 3

Immutability

The standard _.set and merge mutate the object you pass in. Their fp equivalents never do — they return a new value and leave your original data untouched:

var original = { a: { b: 1 } };
var updated = fp.set('a.b', 99, original);

original.a.b;
// => 1  (untouched)

updated.a.b;
// => 99

This makes fp methods safe to use with state you don't own, such as values kept in a Redux store or React state.

Composing functions

fp.flow runs a list of functions left to right, passing each function's result to the next — useful for describing a pipeline as data instead of nested calls:

var pipeline = fp.flow(
  fp.filter(function (n) { return n % 2 === 0; }),
  fp.map(function (n) { return n * 10; })
);

pipeline([1, 2, 3, 4, 5, 6]);
// => [20, 40, 60]

fp.flowRight (aliased as fp.compose) runs the same list right to left, matching traditional mathematical function composition.

When to reach for lodash/fp

Prefer the standard build for straightforward, one-off transformations — its data-first order reads naturally and its examples throughout this reference use it directly. Reach for lodash/fp when you're composing reusable pipeline functions ahead of time, passing partially applied functions as callbacks, or need a guarantee that a Lodash call will never mutate a value you pass in.

Next steps

• Compare this to Lodash's other data-first, mutation-aware methods in the API reference.
• See Installation for other ways to load the fp build, including per-method imports under lodash/fp/*.