list.js

let list = [];

function highlight(){
    alert('Your word is already in the list..');
}


function add_to_list(list){
    let Selection = document.getElementById('selection').value;
    for(let i = 0; i < list.length; i++){
        for(x in list[i]){
            if(x === ''){
                alert('Field is empty');
            }
            if(x === Selection){
                highlight();
            }
            if(x != Selection){
                list.push(x);
            }
        }
    }
}

function remove_from_list(list){
    let Selection = document.getElementById('selection').value;
    for(let i = 0; i < list.length; i++){
        for(x in list[i] == Selection){
            list.pop(x);
        }
    }
}

function view(list){
    console.log(list);
}


view.onclick = function(){
    view(list);
}

push.onclick = function(){
    add_to_list(list);
}

pop.onclick = function(){
    remove_from_list(list);
}