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Implement groupBy(array, iteratee) — lodash style

Q: Implement groupBy(array, iteratee) — lodash style

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the real, defining ordering claim this question tests — that elements WITHIN each group appear in their original, real insertion order, never resorted — was verified directly: a deliberately out-of-order input (`[3,1,2,4]`) produced groups with each element still in its own original relative order. ## 1. The problem, restated Group array elements into a plain object, where each key is the STRINGIFIED result of calling `iteratee` on an element, and each value is a real array of every element that produced that key — preserving each element's own original, relative insertion order within its group, matching lodash's own well-known `_.groupBy` contract, including its real, dual support for a function OR a plain string shortcut. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Function AND string-shortcu

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The problem

Problem, as an interviewer would state it: "Implement groupBy(array, iteratee) — grouping array elements into an object keyed by the result of calling iteratee on each element, matching lodash's own well-known _.groupBy behavior."

Examples:

Code

groupBy([1,2,3,4,5,6], n => n % 2 === 0 ? "even" : "odd");
// { odd: [1,3,5], even: [2,4,6] }

Clarifying questions expected:

Should the iteratee support both a real function AND a plain string shortcut (grouping by that property name directly), matching lodash's own dual-input convention?
What real, correct order should elements appear in WITHIN each group?
What should happen if the iteratee genuinely returns a non-string value, like a number or an object?

Code / implementation expected: Yes — real, direct proof with both a function iteratee and a string-property iteratee, confirming correct grouping and preserved insertion order within each group.

object
groupBy
array

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Easy

How to read this doc: the real, defining ordering claim this question tests — that elements WITHIN each group appear in their original, real insertion order, never resorted — was verified directly: a deliberately out-of-order input ([3,1,2,4]) produced groups with each element still in its own original relative order.

1. The problem, restated

Group array elements into a plain object, where each key is the STRINGIFIED result of calling iteratee on an element, and each value is a real array of every element that produced that key — preserving each element's own original, relative insertion order within its group, matching lodash's own well-known _.groupBy contract, including its real, dual support for a function OR a plain string shortcut.

2. Clarifying questions

The solution is hidden so you can try first

6 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: groupBy correctly partitions by both a function and a string-property iteratee, preserving original order within each group

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