Why should you care?

They let you write flexible, reusable code instead of copy-pasting similar logic.

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.

function applyTwice(fn, value) {
  return fn(fn(value));
}
const addOne = (n) => n + 1;
console.log(applyTwice(addOne, 5)); // 7

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What happened?

• applyTwice receives a function fn and calls it twice.
• addOne is passed as the function argument.

Practice next

1. Run applyTwice with addOne.
2. Use [1,2,3].map(n => n * 2) and log the result.
3. Write filter to keep even numbers only.
4. Write once(fn) that runs fn only the first time it is called.
5. Create compose(f, g) that returns x => f(g(x)).

Remember

Functions are values you can pass around map/filter are higher-order Enables reusable patterns

ScriptVerse data pipeline

Admin exports pass CSV rows through a chain of higher-order map and filter functions.

Outcome: Reusable transform functions keep ETL logic testable and composable.