Why should you care?

Fewer side effects mean fewer bugs in large codebases — popular in React and data pipelines.

See it live — copy this example

Paste into an HTML file or the browser console (F12). Use Run below when the live editor is available.

const nums = [1, 2, 3];
const doubled = nums.map((n) => n * 2);
console.log(doubled);
console.log(nums);

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• map returns a new array; nums is unchanged.
• Pure functions avoid surprising changes elsewhere.

Practice next

1. Run map example and confirm nums is unchanged.
2. Chain map then filter on one array.
3. Compare mutating sort() vs [...nums].sort().
4. Use reduce to sum an array of order totals.
5. Replace a for-loop mutation with map returning new objects.

Remember

Prefer pure functions map/filter return new arrays Immutability reduces bugs

ScriptVerse pricing engine

Cart line items flow through pure map functions that apply discounts without mutating the original catalog.

Outcome: Immutable transforms make undo, audit, and time-travel debugging easier in complex UIs.