Pure and impure, side by side
A pure function always returns the same output for the same inputs and changes nothing outside itself; everything else a function does to the outside world is a side effect, and the IIFE is an older pattern that wrapped code in a function just to keep its variables private.
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 6 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Make this function pure: let count = 0; function next() { count++; return count; } (hint: take the current count as a parameter).
Write a pure applyDiscount(product, percent) that returns a new product object with a reduced price, leaving the original untouched.
Rewrite this IIFE with a block and const: (function () { var secret = 42; console.log(secret); })();
Explain it without notes
Define a pure function and give one example of a pure and one of an impure function.
List five kinds of side effects a JavaScript function can have.
Why can a function change an array passed to it, and how do you avoid it?
What is an IIFE, why was it widely used, and what replaces it today?
Console
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countedDouble ran 2 times