Why using Math.max as a parameter does not work on this scenario? [duplicate]

Question

I was answering a question related to Array.reduce by using Math.max in my example and I found something that I don't understand:

This works:

let values=[4,5,6,77,8,12,0,9];

let max=values.reduce((acc,curr) => Math.max(acc,curr),0);

console.log(max);

But if I try something like this:

let values=[4,5,6,77,8,12,0,9];

let max=values.reduce(Math.max,0);

console.log(max);

It returns NaN.

I thought that the context was the reason, so I wrote the following:

let max=Math.max;
console.log(max(2,5));

But it worked as expected!

What am I missing? MDN says that:

If at least one of the arguments cannot be converted to a number, NaN is returned.

Answer 1

What you're missing is that the callback to reduce has more parameters than just the accumulator and current array value. It actually has 4.

See the docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce#Description

The four parameters:

accumulator
currentValue
currentIndex
array (a reference to the array itself)

The problem is the fourth parameter, it's the array itself that reduce was called on. Math.max can't handle arrays, so it returns NaN.

EDIT: instead, you can use the apply method or the new spread operator!

let values = [4,5,6,77,8,12,0,9];
let max = Math.max.apply(null, values);
let maxAnotherWay = Math.max(...values);

OR, if you happen to be using Lodash, the _.ary method lets you wrap functions in another function that limits its arity:

let values = [4,5,6,77,8,12,0,9];
let max = values.reduce(_.ary(Math.max, 2),0);