What is the difference between slice and splice?

Q: What is the difference between slice and splice?

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every before/after array state below was actually run in Node. ## 1. Why This Even Matters — A Story First Photocopying a few pages out of a physical book (slice) leaves the original book completely intact on the shelf — you walk away with a separate, independent copy. Physically tearing those same pages OUT of the book (splice) leaves the original book genuinely changed forever — fewer pages, and you walk away holding the torn-out pages themselves as a separate bundle. Both actions can hand you back "the same pages," but only one of them damages the original. ## 2. The Core Idea 📌 **Interview term:** `slice` returns a new array (a shallow copy of a range), never touching the original. `splice` mutates the original array in place — removing, inserting, or

The problem

Question presented to candidate: "If you call arr.slice(1, 3) versus arr.splice(1, 2) on the same array, what happens to the ORIGINAL array in each case? Are the return values similar too?"

What a strong answer should cover:

📌 Interview term: slice(start, end) — returns a new array containing a shallow copy of the specified portion, without mutating the original array at all.
📌 Interview term: splice(start, deleteCount, ...items) — mutates the original array in place, removing deleteCount elements starting at start, optionally inserting new items there, and returns a new array of the elements that were removed.
📌 Interview term: the real, direct answer to the prompt — verified directly: after arr.slice(1, 3), the original array was genuinely completely unchanged; after the same-shaped arr.splice(1, 2), the original array was genuinely mutated — the two elements were removed in place, shifting the remaining elements down. slice's return value is the extracted portion; splice's return value is also the extracted portion, but the ORIGINAL array itself is left different afterward.
A precise answer names splice's dual/triple capability, verified directly: passing 0 as deleteCount with additional items makes it insert-only (nothing removed); passing a non-zero deleteCount alongside new items makes it replace elements in place.
A precise answer names that splice has no string equivalent — verified directly, typeof "hello".splice is genuinely undefined — since strings are immutable and splice's entire purpose is in-place mutation; slice works identically on both arrays and strings, since it never needs to mutate anything.

Clarifying questions expected:

None — this is a definitional/comparison question; directly answering the prompt's own mutation question for both methods is the strong signal.

Code / implementation expected: Yes — the before/after array comparison for both methods on identically-shaped calls is the clearest, most convincing demonstration.

array-methods

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript fundamentals interviews. Difficulty: Easy

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every before/after array state below was actually run in Node.

1. Why This Even Matters — A Story First

Photocopying a few pages out of a physical book (slice) leaves the original book completely intact on the shelf — you walk away with a separate, independent copy. Physically tearing those same pages OUT of the book (splice) leaves the original book genuinely changed forever — fewer pages, and you walk away holding the torn-out pages themselves as a separate bundle. Both actions can hand you back "the same pages," but only one of them damages the original.

2. The Core Idea

📌 Interview term: slice returns a new array (a shallow copy of a range), never touchin

The solution is hidden so you can try first

6 min read · 8 sections

Run the code

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

JavaScriptReal, side-by-side proof: slice never mutates the original array while splice genuinely does, plus insert and replace variants