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Custom String.prototype.trim polyfill

Q: Custom String.prototype.trim polyfill

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real, easy-to-get-subtly-wrong detail this question tests — that ONLY leading and trailing whitespace is removed, with INTERNAL whitespace genuinely preserved — was verified directly: `" a b c ".myTrim()` correctly produced `"a b c"`, with the multiple internal spaces between `a`/`b`/`c` genuinely, completely untouched. ## 1. The problem, restated Remove every whitespace character (spaces, tabs `\t`, newlines `\n`, and other real Unicode whitespace) from ONLY the leading and trailing edges of a string, genuinely preserving any internal whitespace exactly as-is — matching real native `String.prototype.trim`'s own complete, documented contract, including its correct coercion of a non-string `this` via `String(this)`. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Tabs and newlines inclu

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77 of 154

The problem

Problem, as an interviewer would state it: "Implement String.prototype.myTrim from scratch — removing leading AND trailing whitespace, matching real native trim's own complete, documented definition of whitespace (spaces, tabs, newlines, and other Unicode whitespace characters), while preserving any internal whitespace."

Examples:

Code

"  hello world  ".myTrim(); // "hello world"

Clarifying questions expected:

Does "whitespace" for this purpose genuinely need to include tabs and newlines, not just plain spaces?
Should INTERNAL whitespace (between words) be genuinely preserved, only the LEADING and TRAILING whitespace removed?
Should this correctly coerce a non-string this (via String(this)), matching real, generic string-method behavior?

Code / implementation expected: Yes — real, direct proof matching real native trim across spaces, tabs, and newlines, confirming internal whitespace is genuinely preserved.

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-get-subtly-wrong detail this question tests — that ONLY leading and trailing whitespace is removed, with INTERNAL whitespace genuinely preserved — was verified directly: " a b c ".myTrim() correctly produced "a b c", with the multiple internal spaces between a/b/c genuinely, completely untouched.

1. The problem, restated

Remove every whitespace character (spaces, tabs \t, newlines \n, and other real Unicode whitespace) from ONLY the leading and trailing edges of a string, genuinely preserving any internal whitespace exactly as-is — matching real native String.prototype.trim's own complete, documented contract, including its correct coercion of a non-string this via String(this).

2. Clarifying questi

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: the trim polyfill matches real native trim, confirming internal whitespace is genuinely preserved while only leading/trailing whitespace is removed

Practice

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