Reusable code

Do more with functions

Lesson 6 of 20 · 7 min · Module 2: Decisions & reusable work

A function is a recipe you can use again and again. Give it an input, let it do its thing, and get an output.

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By the end, you’ll be able to…

Define parameters and a return value
Call the same function with different inputs
Apply a function to every array item

The idea

Parameters are the names for inputs. The return keyword sends a value back. An array’s map() method runs a function for each item.

function square(number) {
  return number * number;
}
console.log(square(4)); // 16

01A parameter is a local name

In double(number), number is supplied by the caller. Returning number * 2 works with different inputs; returning a fixed value only solves one example.

function double(number) { return number * 2; }
console.log(double(3));
console.log(double(5));

Output

6
10

02Pass a function, not one result

numbers.map(double) gives map the function to call for each item. numbers.map(double()) would call double immediately without an argument and pass its result instead.

Your turn

Write double(number) to return twice its input. Then create doubled by doubling each number in [1, 2, 3].

What the checks look for

double works for positive numbers
double handles zero and negatives
doubled contains [2, 4, 6]

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Take this with you

A reusable function describes a relationship between inputs and outputs.