Lodash intersectionBy with Undefined Iteratee

When the iteratee function provided to Lodash's _.intersectionBy returns undefined for every element, the method collapses all elements into a single shared comparison key. Assuming all provided arrays contain at least one element, _.intersectionBy treats every element across all arrays as equivalent and returns an array containing exclusively the first element of the first array. If any provided array is completely empty, the method returns an empty array.

How Lodash Evaluates the Iteratee

The _.intersectionBy method computes the intersection of multiple arrays based on a criterion generated by invoking an iteratee for each element. The values in the resulting array are drawn directly from the first array, while subsequent arrays dictate which criteria are preserved.

Internally, Lodash generates mapped criteria sets for comparison:

1. Each element in the primary array is mapped to a criterion using the iteratee.
2. Each element in the secondary arrays is similarly mapped to build lookup sets.
3. Elements from the first array are retained only if their computed criterion exists within all secondary arrays.
4. Duplicate criteria are discarded to ensure the resulting array contains unique matches based on the criterion.

Why the Result Contains Only the First Element

When the iteratee explicitly or implicitly returns undefined for every element:

• Criterion Homogeneity: Every element in every array produces the identical criterion: undefined.
• Intersection Match: Lodash checks whether the first element of the first array (whose criterion is undefined) exists in the criteria of all other arrays. Because the other arrays also map to undefined, a match is confirmed.
• Deduplication: Lodash maintains a cache of already-matched criteria to prevent duplicate elements in the intersection. Once the criterion undefined is added to this cache via the first element, every subsequent element in the first array—which also maps to undefined—is flagged as a duplicate and ignored.

Consequently, only the very first element of the first array passes into the final result.

Example Behavior

const _ = require('lodash');

const array1 = ['apple', 'banana', 'cherry'];
const array2 = ['orange', 'grape'];
const array3 = ['melon'];

// Iteratee returns undefined for every element
const result = _.intersectionBy(array1, array2, array3, () => undefined);

console.log(result);
// Output: ['apple']

In the event that any array passed to the method is empty:

const emptyArray = [];
const resultWithEmpty = _.intersectionBy(array1, emptyArray, () => undefined);

console.log(resultWithEmpty);
// Output: []

Because emptyArray contains no elements, the criterion undefined is never produced for that array, meaning no common criterion exists across all inputs.