Index of the biggest value in an array

Let's say we have this array of years:

const years = [1941, 1714, 2005, 1998, 1871, 2001, 1700]

How do we get the index of the biggest value in the array?

A common approach is to reach for a regular for-loop.

const years = [1941, 1714, 2005, 1998, 1871, 2001, 1700]

let max = years[0]
let maxIndex = 0

for (let i = 1; i < years.length; i++) {
	if (years[i] > max) {
		maxIndex = i
		max = years[i]
	}
}

// maxIndex = 2

It works, but as usual I think for-loops are a bad solution for most things. The solution requires several mutable variables (max, maxIndex, i) and just having an index variable that keeps track of how many iterations to do feels outdated.

Apart from the for-loop, the natural way is probably to first get the max value and then check where in the array that value is. Like this:

const years = [1941, 1714, 2005, 1998, 1871, 2001, 1700]

const maxYear = Math.max(...years)

// maxYear = 2005

const maxYearIndex = years.indexOf(maxYear)

// maxYearIndex = 2

This solution requires the years array to be looped through twice though — first when computing the max value, then when finding the index of the max value. Isn't there a nice solution without unnecessary computation?

While thinking about a way to get the index without the intermediate step of computing the max value, I came across this solution:

const years = [1941, 1714, 2005, 1998, 1871, 2001, 1700]

const maxYearIndex = years.reduce((maxIndex, curr, index, arr) => {
	return curr > arr[maxIndex] ? index : maxIndex
}, 0)

// maxYearIndex = 2

You can't say the code got more readable, but it's probably one of the best solutions performance-wise. Clean code is always a trade-off between performance and readability — it's rarely justified to make a really slow and blunt solution just because it's faster to understand.

You don't often get to use all four arguments to the reduce callback either. Extra nice!

To make the code clearer, the complex part can be extracted into its own function:

const years = [1941, 1714, 2005, 1998, 1871, 2001, 1700]

const maxYearIndex = getMaxYearIndex(years)

// maxYearIndex = 2

function getMaxYearIndex(array) {
	return array.reduce((maxIndex, curr, index, arr) => {
		return curr > arr[maxIndex] ? index : maxIndex
	}, 0)
}

There we go!