MergeSort in Javascript
I don't see a way to improve the algorithm, but here are some general coding tips:

A number can either be greater than 1, or less than 2. So you can use 'else' here instead.

if(list.length &lt; 2){
    return list
}else if(list.length &gt; 1){


Sometimes people create extra variables to improve readability, but here 'n' is less readable than list.length.

let n = list.length


A little bit of commenting can go a long way, for example if you could explain why you add 'Infinity' to the lists in a comment:

listA.push(Infinity)
listB.push(Infinity)