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Get random element from an array (JavaScript)

function randomItem(arr) {
  return arr[Math.floor(Math.random() * arr.length)];
}

randomItem(['a', 'b', 'c']); // 🤷 'a' or 'b' or 'c', it's random

How does it work?

  1. Get a random number between 0 and 1 with Math.random. It can return 0 but never 1.
  2. Multiply the random number from point 1 by the number of items on the array.
  3. Use Math.floor to remove the decimal part.

Notice that you cannot use Math.round instead of Math.floor because it could round up and return an array index out of bounds.

How could someone come up with this solution?

The Math.random method can return all the values between 0 and almost 1.

Promises diagram

What happens if we floor it? We can only get the value 0.

OK, but let's say we have a list with 10 elements? In that case we need values between 0 and 9.

If we multiply Math.random by 10 we get values between 0 and almost 10.

Promises diagram

If we floor it we get the integers from 0 up to 9:

Promises diagram

Note: On the previous charts the x axis are tries, y axis are all the possible random values. We have to assume we selected and sorted the tries to get all the possible random values.

Learn more about discrete vs continuous values.

Questions

What's the function f output?

function f(items) {
  return items[Math.floor(Math.random() * items.length)];
}

f([1, 2, 3]);