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Implement _.range(start, end, step) generator

Q: Implement _.range(start, end, step) generator

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the "generator can represent infinity, array cannot" claim is proven directly below by actually lazily consuming 5 values from a real infinite generator — not just asserted as a theoretical advantage. ## 1. The problem, restated Produce the sequence `start, start+step, start+2*step, ...` stopping before reaching `end` (exclusive), supporting a negative `step` for counting down. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Return an array, or something lazily iterable? | The core design decision this question is testing — determines whether huge or infinite ranges are even representable. | | Negative step supported? | Confirms the loop condition needs to flip direction based on `step`'s own sign, not assume `start < end`. | | `end` inclusive or exclusive? | Matches real lodash's own documented exc

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The problem

Problem, as an interviewer would state it: "Write a range(start, end, step) like lodash's — produces the sequence of numbers from start up to (not including) end, stepping by step. Then tell me a real reason to implement it as a generator instead of returning a plain array."

Examples:

Code

[...range(0, 10, 2)] -> [0, 2, 4, 6, 8]
[...range(10, 0, -2)] -> [10, 8, 6, 4, 2]

Clarifying questions expected:

Should this return a real array directly, or something lazily iterable (a generator)?
Is a negative step expected to work symmetrically (counting down)?
Should end ever be included, or always strictly exclusive like the real lodash behavior?

Code / implementation expected: Yes — real, direct proof that a generator-based range can represent a genuinely infinite sequence, which an array-materializing version structurally cannot.

range
array
utility

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Solution

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

How to read this doc: the "generator can represent infinity, array cannot" claim is proven directly below by actually lazily consuming 5 values from a real infinite generator — not just asserted as a theoretical advantage.

1. The problem, restated

Produce the sequence start, start+step, start+2*step, ... stopping before reaching end (exclusive), supporting a negative step for counting down.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Return an array, or something lazily iterable?	The core design decision this question is testing — determines whether huge or infinite ranges are even representable.
Negative step supported?	Confirms the loop condition needs to flip direction based on `step`'s own sign, no

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5 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: a generator-based range() creates instantly and can represent a genuinely infinite sequence, unlike an array-materializing version

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