How Lodash maxBy Extracts Properties

The Lodash _.maxBy method computes the maximum value of an array based on criteria generated by an iteratee. When a property name or path is supplied instead of a custom callback function, Lodash automatically resolves it using its built-in _.property iteratee shorthand. This article breaks down how _.maxBy extracts properties from objects, how the underlying baseIteratee works, and how to utilize different extractor patterns in your code.

The Property Extractor: _.property

When you invoke _.maxBy(array, [iteratee=_.identity]) and pass a string as the second argument, Lodash does not evaluate the string directly. Instead, it converts the string into a getter function via _.property(path).

This extractor navigates each object in the collection, retrieves the value associated with the specified key, and uses that value to perform numerical comparisons using standard greater-than (>) logic.

const _ = require('lodash');

const users = [
  { name: 'Alice', score: 84 },
  { name: 'Bob', score: 95 },
  { name: 'Charlie', score: 90 }
];

// Lodash converts 'score' into _.property('score')
const topUser = _.maxBy(users, 'score');
// Output: { name: 'Bob', score: 95 }

How Lodash Resolves Iteratees Internally

The property extraction mechanism in _.maxBy is powered by an internal Lodash helper called baseIteratee. Whenever a method in Lodash accepts an iteratee argument, baseIteratee inspects the data type:

• String or Array Path: Converts the argument into _.property(path), creating a function that retrieves shallow or deeply nested values.
• Object: Converts the argument into _.matches(source), checking for matching key-value pairs.
• Function: Uses the custom function directly without modification.
• Undefined/Null: Defaults to _.identity, which evaluates the elements themselves.

Because of this system, passing a string like 'score' to _.maxBy(users, 'score') is functionally identical to writing _.maxBy(users, _.property('score')) or _.maxBy(users, o => o.score).

Deep Path Property Extraction

The _.property extractor supports nested paths. You can pass dot-notation strings or arrays of keys to extract properties located deep within nested object structures without risking TypeError: Cannot read properties of undefined errors.

const inventory = [
  { item: 'Laptop', pricing: { retail: 1200 } },
  { item: 'Monitor', pricing: { retail: 300 } },
  { item: 'Keyboard', pricing: { retail: 100 } }
];

// Dot notation
const priciestItem = _.maxBy(inventory, 'pricing.retail');

// Array path notation
const alsoPriciestItem = _.maxBy(inventory, ['pricing', 'retail']);

In both cases, Lodash’s deep property getter safely traverses the structure. If an object lacks the specified path, the extractor returns undefined, which is ignored during ranking comparisons.