Why Math.max(...[]) is equal to -Infinity in ES2015?

If you see the internal implementation documentation, you can tell why Math.max is returning -Infinity when there is no argument passed.

If no arguments are given, the result is -∞.

So when you spread an empty array in a function call, it is like calling the function without an argument.

If you look at the babel output for Math.max(...[]), you end up with Math.max.apply(Math, []). If you log that in ES5, you see that for some reason it gives you -Infinity, because it's the same as calling it without an argument.

And indeed, Math.max() gives -Infinity

If you need a reminder: fn.apply( yourThis, [ a, b, c ] ) is the same as fn( a, b, c )

What happens with Math.max([]) is that [] is first converted to a string and then to a number. It is not actually considered an array of arguments.

With Math.max(...[]) the array is considered a collection of arguments through the spread operator. Since the array is empty, this is the same as calling without arguments. Which according to the docs produces -Infinity

If no arguments are given, the result is -Infinity.

Some examples to show the difference in calls with arrays:

console.log(+[]); //0    [] -> '' -> 0
console.log(+[3]); //3    [] -> '3' -> 3
console.log(+[3,4]); //Nan 
console.log(...[3]); //3
console.log(...[3,4]); //3 4 (the array is used as arguments)
console.log(Math.max([])); //0  [] is converted to 0
console.log(Math.max()); // -infinity:  default without arguments
console.log(Math.max(...[])); // -infinity
console.log(Math.max([3,4])); //Nan
console.log(Math.max(...[3,4])); //4