Why Lodash Uses _.identity as a Default Iteratee

In the Lodash JavaScript library, _.identity is a utility function that simply returns the first argument it receives. This article explores why Lodash uses _.identity as the default iteratee across many of its array and collection methods. By acting as a neutral pass-through function, _.identity provides a dependable fallback that standardizes functional logic, eliminates repetitive callback code, and simplifies operations like filtering, mapping, and deduplication.

The Role of _.identity

In JavaScript, _.identity is defined essentially as:

function identity(value) {
  return value;
}

Its primary purpose is to serve as a mathematical identity function: \(f(x) = x\). While it performs no actual mutation or computation on the input, its power lies in its ability to be passed as a first-class function where an action is syntactically required but logically unnecessary.

Avoiding Branching Logic in the Source Code

Higher-order functions in Lodash—such as map, filter, uniqBy, groupBy, and sortBy—rely on an iteratee to process each element in a collection.

Without a default function, the library's internal implementation would require conditional checks inside loops to determine whether an iteratee was provided by the user:

// Inefficient approach with conditional checks
for (let i = 0; i < array.length; i++) {
  const value = iteratee ? iteratee(array[i]) : array[i];
  // Process value...
}

By assigning _.identity as the default iteratee fallback, Lodash avoids branching conditionals inside performance-critical iteration loops. The internal code can execute iteratee(array[i]) unconditionally, keeping the engine's execution path optimized and the codebase uniform.

Natural Truthy Filtering

One of the most practical applications of _.identity as a default iteratee is in _.filter. If no predicate function is supplied, _.filter defaults to _.identity.

Because JavaScript evaluates return values in logical contexts, passing an element to _.identity returns the element itself. The filter then coerces that element to a boolean:

const mixedData = [0, 'hello', false, 42, '', null, undefined];
const truthyValues = _.filter(mixedData); 
// Result: ['hello', 42]

This default eliminates the need to pass explicit functions like Boolean or (x) => !!x to clear falsy elements from a list.

Seamless Pass-Through for Sorting and Deduplication

Methods like _.uniqBy and _.sortBy evaluate elements based on the criteria produced by their iteratee. When using these methods on primitive types (like numbers or strings), developers usually want to compare the items directly rather than inspect a property.

Setting _.identity as the default means that functions requiring an iteratee can automatically degrade gracefully to handle primitive values:

Functional Composition and Predictability

Lodash heavily adheres to functional programming principles. Higher-order functions are expected to maintain consistent signatures. By using _.identity as a baseline, Lodash ensures that functions always accept and invoke a transformer, making function composition, currying, and point-free programming paradigms consistent across the entire API.