Lesson overview · Free interview practice

Higher-Order Functions

Master functions as first-class citizens and essential array methods.

Topics in the full lesson
  • Functions as First-Class Citizens
  • Higher-Order Functions
  • Callbacks
  • Array Methods: Map, Filter, Reduce
  • Best Practices
  • Exercises

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TheoryEasy

What is a Higher-Order Function

Question

What is a higher-order function (HOF)? Show both forms with code, and explain why HOFs are common in JavaScript.

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TheoryEasy

First-Class Functions

Question

What does "first-class functions" mean in JavaScript? Show the four capabilities that follow from it.

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TheoryEasy

`map` vs `forEach`

Question

How does map differ from forEach? When should you reach for each?

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TheoryMedium

How `reduce` Works

Question

Explain how reduce works. Show the canonical patterns: reduce-to-value and reduce-to-structure.

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TheoryHard

Implement `debounce`

Question

Implement debounce(fn, wait) — a HOF that returns a function which delays calling fn until wait ms have passed since the last invocation. Walk through how it works.

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