Javascript prototype sorting function don't compare with all members

while I was cracking the leet code task I bumped into some unexpected behavior of the .sort function, so I couldn’t finish my task until I implemented the insertion sorting function myself which helped me to sort an array of chars properly.

So the question:
why this function doesn’t compare with all the array members?

function customSortString(order: string, s: string): string {
    const map={};
    for(let i=0;i<order.length;i++){
        map[order[i]]=i;
    }
    const newStr=[...s].sort((a,b)=>{
        if(map[a]===undefined || map[b]===undefined || a===b){
            return 0;
        }
        if(map[a]<map[b]){
            return -1;
        }
        return 1;
    })
    return newStr.join('');
    
};

if I call

customSortString("exv","xwvee")

it will return ‘xweev’, but expected "eexvw"

here is a description of the problem

If I put console.log into to sorting function

w x
v w
e v
e w
e v
e e
e v

So I can see, that "x" is not compared with "e". But if it did compare it would return 1 and would display "e" first then "x"

I hope I explained clearly, but ask questions if you didn’t

>Solution :

You need to move unknown characters to the end, with a large value.

function customSortString(order, s) {
    const map = {};
    for (let i = 0; i < order.length; i++) map[order[i]] = i + 1;
    return [...s]
        .sort((a, b) => (map[a] || Infinity) - (map[b] || Infinity))
        .join('');
};

console.log(customSortString("exv", "xwvee"));