Once I was debugging, I found that in Firefox Array.sort() can take boolean values in addition to number values. However, in Node.js Array.sort() only supports number values. Although it didn’t cause bugs, I’m curious about the difference.

Difference between JS engines

Here are some tests:

• Firefox

const arr = [{num:1}, {num:3}, {num:2}]
const numSorted = arr.sort((a, b) => a.num - b.num)
console.log(numSorted)

> Array [Object { num: 1 }, Object { num: 2 }, Object { num: 3 }]

const arr = [{num:1}, {num:3}, {num:2}]
const boolSorted = arr.sort((a, b) => a.num > b.num)
console.log(boolSorted)

> Array [Object { num: 1 }, Object { num: 2 }, Object { num: 3 }]

• Node.js

const arr = [{num:1}, {num:3}, {num:2}]
const sorted = arr.sort((a, b) => a.num > b.num)
console.log(sorted)

> [ { num: 1 }, { num: 3 }, { num: 2 } ]

• Deno

const arr = [{num:1}, {num:3}, {num:2}]
const sorted = arr.sort((a, b) => a.num > b.num)
console.log(sorted)

> [ { num: 1 }, { num: 3 }, { num: 2 } ]

It turns out that in Node.js and Deno (both use V8), Array.sort() takes only numbers. While in Firefox, Array.sort() can take both.

Array.sort() in TypeScript

In TypeScript, Array.sort() is

(method) Array<T>.sort(compareFn?: ((a: T, b: T) => number) | undefined): T[]

Try to use boolean values in Array.sort(), but Typescript throws error:

Argument of type '(a: { num: number; }, b: { num: number; }) => boolean' is not assignable to parameter of type '(a: { num: number; }, b: { num: number; }) => number'.
  Type 'boolean' is not assignable to type 'number'.ts(2345)

Array.sort() in MDN and ECMA

Let’s check out the document on MDN¹, it says

So, the compare function has the following form:

function compare(a, b) {
 if (a is less than b by some ordering criterion) {
   return -1;
 }
 if (a is greater than b by the ordering criterion) {
   return 1;
 }
 // a must be equal to b
 return 0;
}

The document doesn’t indicate the sort() can take boolean values. Continue to check out the ECMA standard², which says

1. If x and y are both undefined, return +0𝔽.
2. If x is undefined, return 1𝔽.
3. If y is undefined, return -1𝔽.
4. If comparefn is not undefined, then
   • Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
   • If v is NaN, return +0𝔽.
   • Return v.
5. Let xString be ? ToString(x).
6. Let yString be ? ToString(y).
7. Let xSmaller be ! IsLessThan(xString, yString, true).
8. If xSmaller is true, return -1𝔽.
9. Let ySmaller be ! IsLessThan(yString, xString, true).
10. If ySmaller is true, return 1𝔽.
11. Return +0𝔽.

It also doesn’t point out the sort() can take boolean values.

Summary

Array.sort() in Firefox can take boolean values, but there is no relevant description in documents. It may just be an implementation difference that probably not causing bugs.