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Custom Object.assign polyfill

Q: Custom Object.assign polyfill

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real, easy-to-miss detail this question tests — that `Object.assign` genuinely MUTATES and returns the SAME target object reference, rather than creating a new one — was verified directly: a real reference-equality check (`result === target`) confirmed this, matching real native `Object.assign` exactly. ## 1. The problem, restated Copy the OWN ENUMERABLE properties (both string and Symbol keys) from one or more source objects onto a single target object, processing sources LEFT TO RIGHT so that a later source's value for a shared key correctly overwrites an earlier one — mutating and returning the SAME target reference, matching real native `Object.assign`'s own complete, documented contract. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Last source wins on key conflicts? | Yes — real native

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72 of 154

The problem

Problem, as an interviewer would state it: "Implement Object.assign-equivalent behavior from scratch — merging one or more source objects into a target, matching real native Object.assign's own documented contract, including which object reference it actually returns."

Examples:

Code

Object.assign({a: 1}, {b: 2}, {a: 3, c: 4}); // {a: 3, b: 2, c: 4}

Clarifying questions expected:

When multiple sources define the same key, does the LAST source genuinely win, matching a real, predictable left-to-right merge order?
Does the target object get MUTATED in place, or is a genuinely new object returned?
Should enumerable Symbol-keyed properties on a source be copied too, not just string keys?

Code / implementation expected: Yes — real, direct proof of correct left-to-right merge-and-overwrite order, that the SAME target reference is mutated and returned, and that enumerable Symbol-keyed properties are correctly copied too.

js-coding
utility
algorithm

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Solution

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

How to read this doc: the real, easy-to-miss detail this question tests — that Object.assign genuinely MUTATES and returns the SAME target object reference, rather than creating a new one — was verified directly: a real reference-equality check (result === target) confirmed this, matching real native Object.assign exactly.

1. The problem, restated

Copy the OWN ENUMERABLE properties (both string and Symbol keys) from one or more source objects onto a single target object, processing sources LEFT TO RIGHT so that a later source's value for a shared key correctly overwrites an earlier one — mutating and returning the SAME target reference, matching real native Object.assign's own complete, documented contract.

2. Clarifying questions a strong candidate asks

The solution is hidden so you can try first

7 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: the Object.assign polyfill matches real native output, mutates and returns the same target reference, and copies enumerable symbol keys correctly

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