The goal is to implement a difference function, which subtracts one list from another.

array-diff.js

function array_diff(a, b) {
  
  // This here is the wwrong approach where the length keeps decresing so 
  // the map function wouldn't check all the arrays element 
  a.map(function(element){
    for(var i =0; i <b.length; i++){
      if(element === b[i]){
         a.splice(a.indexOf(element),1);
         break;
      }
    }});
  return a;
}

// a better approach where you filter the array 
// and the condition is that the element in a 
// returns a -1 in array b meaning it doesnt exist there
function array_diff(a, b) {
  return a.filter(function(x) { return b.indexOf(x) == -1; });
}

Thank you @ReemAlJunaid it worked! Thanks also for pointing out the length property change. it didn't cross my mind so I thank you for that. With more research, I found a better solution a one liner check it out, I think it's pretty cleaver
function array_diff(a, b) { return a.filter(function(x) { return b.indexOf(x) == -1; }); }