When implementing a hash map it doesn't work but works when use an object

I am new to programming and hash data structure. I was trying to solve this problem. It works when I used an Object as the codes showed below. But it fails to work when I use a Map. Could someone kindly tell me what went wrong with my map? Many many thanks!

//with object:
var groupStrings = function(strings) {
    const res = {};
    for(let s of strings){
        const key = new Array(s.length).fill(0);
        for(let i=1;i<s.length;i++){
            key[i]=(s.charCodeAt(i)-s.charCodeAt(i-1)+26)%26;

        }
            if(res[key]){
                res[key].push(s)
                }
        else res[key]=[s] ;  
        }
            return Object.values(res);
};


//The one used map and didn't work. 
/**
 * @param {string[]} strings
 * @return {string[][]}
 */
var groupStrings = function(strings) {
    const map1 = new Map();
    for(let s of strings){
        const key = new Array(s.length).fill(0);
        for(let i=1;i<s.length;i++){
            key[i]=(s.charCodeAt(i)-s.charCodeAt(i-1)+26)%26;
        }
            if(map1.has(key)){map1.get(key).push([s])}
        else map1.set(key,[s]) ;  
        }    
    return [...map1.values()];
};