Top Use Cases for Lodash assignWith

The Lodash library provides the _.assignWith method to extend object merging capabilities by allowing developers to define a customizer function. Unlike standard assignment methods like Object.assign() or Lodash's _.assign(), which blindly overwrite target properties with source values, _.assignWith enables granular control over how conflicting keys are resolved. This article outlines the primary use cases where _.assignWith is the most effective solution, including conditional updates, numerical aggregation, non-destructive array handling, and data sanitization.

1. Merging and Aggregating Numerical Values

In analytical and financial applications, merging two objects often requires calculating sums, differences, or averages rather than replacing the original values. _.assignWith allows you to inspect the existing target value and the incoming source value to compute a new aggregate.

function sumCustomizer(objValue, srcValue) {
  if (typeof objValue === 'number' && typeof srcValue === 'number') {
    return objValue + srcValue;
  }
}

const monthlyUsage = { apiCalls: 1200, storageGB: 50 };
const newUsage = { apiCalls: 300, storageGB: 10 };

const totalUsage = _.assignWith(monthlyUsage, newUsage, sumCustomizer);
// Result: { apiCalls: 1500, storageGB: 60 }

2. Concatenating Arrays Instead of Overwriting

Standard shallow assignment operators replace existing arrays with incoming ones. When combining user roles, tags, or system logs, the desired behavior is frequently concatenation or finding the union of unique entries. _.assignWith intercepts array conflicts to merge them seamlessly.

function arrayUnionCustomizer(objValue, srcValue) {
  if (Array.isArray(objValue) && Array.isArray(srcValue)) {
    return _.union(objValue, srcValue);
  }
}

const defaultSettings = { tags: ['admin', 'moderator'] };
const customSettings = { tags: ['editor', 'admin'] };

_.assignWith(defaultSettings, customSettings, arrayUnionCustomizer);
// Result: { tags: ['admin', 'moderator', 'editor'] }

3. Preventing null or undefined Overwrites

Incoming payloads from forms, external APIs, or partial update requests often contain null or undefined fields for untouched inputs. If combined using native assignment, these empty fields overwrite valid target data. Using _.assignWith guarantees that valid default or existing values are preserved.

function preserveExistingCustomizer(objValue, srcValue) {
  return srcValue === undefined || srcValue === null ? objValue : srcValue;
}

const currentProfile = { name: 'Alice', bio: 'Software Engineer' };
const incomingPatch = { name: 'Alice M.', bio: null };

_.assignWith(currentProfile, incomingPatch, preserveExistingCustomizer);
// Result: { name: 'Alice M.', bio: 'Software Engineer' }

4. Timestamp-Based Conflict Resolution

When synchronizing data across distributed nodes or offline-first clients, objects often carry metadata such as update timestamps or version numbers. _.assignWith can be configured to evaluate version properties and only commit updates if the incoming data is strictly newer than the current state.

function timestampCustomizer(objValue, srcValue, key, object, source) {
  if (object.updatedAt && source.updatedAt) {
    return source.updatedAt > object.updatedAt ? srcValue : objValue;
  }
}

5. Type Coercion and Data Normalization

During ingestion processes, identical keys might arrive with mismatched types, such as strings instead of integers or ISO date strings instead of Date objects. _.assignWith enables inline normalization so the destination object maintains type consistency without requiring separate pre- or post-processing loops.