Math.{minIndex, maxIndex} about proposal-math-extensions HOT 7 OPEN

In mathematics, inf, sup and argmax have very precise definitions (which don't match your test cases above).

Math.minIndex and Math.maxIndex are much more analogous to Math.min and Math.max. This naming is also consistent with Array.prototype.find and Array.prototype.findIndex.

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Rust appears to implement it on iterators, not arrays (based on your example)

It’s a good point that being indexes implies it’s for arrays and nothing else (even generic iterators don’t really have indexes).

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I'd (still) love to have this functionality as well, but as a small nit, it seems somewhat of a constraint to me to define this directly on the Math object? Wouldn't defining it on arrays make it more versatile?

[4, 2, 4, 2].minIndex() === 1
[4, 2, 4, 2].reverse().minIndex() === 0

I often use position_min() and position_max() in Rust, which are defined in a similar manner and are part of the popular Itertools crate.

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These implementations make a single pass through the array

function indexOfMax(array) {
    return array.reduce((a, v, i) => 1 in a && v <= a[1] ? a : [i, v], [-1])[0];
}

function indexOfMin(array) {
    return array.reduce((a, v, i) => 1 in a && v >= a[1] ? a : [i, v], [-1])[0];
}

function lastIndexOfMax(array) {
    return array.reduceRight((a, v, i) => 1 in a && v <= a[1] ? a : [i, v], [-1])[0];
}

function lastIndexOfMin(array) {
    return array.reduceRight((a, v, i) => 1 in a && v >= a[1] ? a : [i, v], [-1])[0];
}

Examples

indexOfMax( [] );                // -1
indexOfMax( [ 4, 2, 4, 2 ] );    // 0
indexOfMax( [ 1n, 1n, 4n ] );    // 2
indexOfMax( [ "a", "b", "c" ] ); // 2

indexOfMin( [] );                // -1
indexOfMin( [ 4, 2, 4, 2 ] );    // 1
indexOfMin( [ 4n, 1n, 4n ] );    // 1
indexOfMin( [ "x", "a", "z" ] ); // 1

I'm not convinced it is worth these being built-in; they could easily be made available in a JavaScript library.

If they are to be built-in, I suggest they be methods on the Array.prototype object rather than functions on the Math object.

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Why should they be methods on Array.prototype

Hi @ljharb! I think of it as a function that perhaps should work on more than just Number and on any value that can be compared to another:

let names = ["a", "d", "c", "b"].maxIndex(); // 1 ("d"), as its code point is U+0044

Rust's Itertools, as precedent, implements it in a similar manner:

use itertools::Itertools;

["a", "d", "b", "c"].iter().position_max() // Some(1)

Array.prototype is also the first place I would look for a method that returns an index, akin to:

• Array.prototype.indexOf()
• Array.prototype.lastIndexOf
• Array.prototype.findIndex

Next to that, I personally find it more ergonomic and easier on the eyes when chaining, e.g:

let values = [ { re: 3, im: 4 }, { re: 8, im: 0 }, { re: 6, im: 8 } ];

const maxAbsIndex = (values) =>
  Math.maxIndex(values.map(({ re, im }) => Math.hypot(re, im)));

const maxAbsIndex = (values) =>
  values.map(({ re, im }) => Math.hypot(re, im)).maxIndex();

I'm not a big fan of the extra level of nesting that comes with Math.maxIndex() and think it would work better for the imperative and functional approach, especially with larger chains in case of the latter.

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// 1 year.. still no Math.maxIndex in sight..
const maxI = a => { const x=Math.max(...a); return a.indexOf(x) };
// or const maxI = a => a.reduce((i, _, j) => a[j]>a[i]?j:i, 0);
// I leave this comment for people in 2025 :p

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Why should they be methods on Array.prototype, when Math.max/Math.min already establishes the precedent for a Math function that takes a list of arguments?

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