'For in' does not recognize the elements in an array

The easiest way to have what you want in a foreach cycle is to change the

in 

for a

of

so that it would be almost as it is.

function isValidWalk(walk) {
    console.log(walk.length);
    if (walk.length ===10) { 
        var WalkN=[]; 
        var WalkS=[];
        var WalkE=[];
        var WalkW=[];
        console.log(walk);
        for (var elemento of walk) { 
            console.log(elemento);
            console.log(walk);
            switch (elemento) { 
                case 'n':
                 WalkN.push(elemento);
                 break;
                case 's':
                 WalkS.push(elemento);
                 break;
                case 'e':
                 WalkE.push(elemento);
                 break;
                case 'w':
                 WalkW.push(elemento);
            }
        console.log( WalkN,WalkS,WalkE,WalkW);
         if (WalkN.length===WalkS.length && WalkE.length===WalkW.length) {
             return true
         }
         else { return false }
        }
    }
    else {
        return false 
    }
}
isValidWalk(['n','s','n','s','n','s','n','s','n','s'])

that way you can access the contents of the array and not the index.

The problem is that for..in is not designed for arrays but for objects. To object to the result you want, it is better to use forEach like this:

function isValidWalk(walk) {
    console.log(walk.length);
    if (walk.length ===10) { 
        var WalkN=[]; 
        var WalkS=[];
        var WalkE=[];
        var WalkW=[];
        console.log(walk);
        walk.forEach(function(elemento) { 
            console.log(elemento);
            console.log(walk);
            switch (elemento) { 
                case 'n':
                 WalkN.push(elemento);
                 break;
                case 's':
                 WalkS.push(elemento);
                 break;
                case 'e':
                 WalkE.push(elemento);
                 break;
                case 'w':
                 WalkW.push(elemento);
            }
            
            if (WalkN.length===WalkS.length && WalkE.length===WalkW.length) {
              return true
            }
            else { return false }
        });
        console.log( WalkN,WalkS,WalkE,WalkW);
    } else {
        return false 
    }
}
isValidWalk(['n','s','n','s','n','s','n','s','n','s'])

You can do it this way too:

var mi_array =  ['n','s','n','s','n','s','n','s','n','s'];
 isValidWalk(mi_array);

 function isValidWalk(walk) {
    if (walk.length == 10) { 
    var WalkN = []; 
    var WalkS = [];
    var WalkE = [];
    var WalkW = [];

    for (var i = 0; i < walk.length; i++){ 

        switch (walk[i]) { 
            case 'n':
             WalkN.push(walk[i]);
             break;
            case 's':
             WalkS.push(walk[i]);
             break;
            case 'e':
             WalkE.push(walk[i]);
             break;
            case 'w':
             WalkW.push(walk[i]);
        }

     if (WalkN.length == WalkS.length && WalkE.length == WalkW.length) {
         return true
     }
     else { return false }
    }
}
else {
    return false 
}
}