javascript: loop through object array and pushing elements into one array

The problem is that you are pushing all of the values for each list[i] in one single operation. This means that you are adding all the elements of each list into a single array slot, which creates an array of arrays.

One thing you can do is to flatten the array after you are done by

c.data.require_field.flatten();

Another option is to individually insert each entry within the list rather than pushing them all in at once:

for(var i=0; i<c.data.list.length; i++) {
   var list = c.data.list[i];

   //for each element in this list, add it to a new spot in the required_fields array
   for(var j=0; j<list.length){
      c.data.required_fields.push(
          c.data.list[j].required_fields.display_value.split(',')             
      );
   }                
}

The first is probably much simpler and succinct, but the second example is added here for clarity on what exactly is going wrong

If you would like to flatten your array there is a code snippet on MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatten

var arr1 = [1, 2, [3, 4]];
arr1.flatten();

//to flatten single level array
arr1.reduce((acc, val) => acc.concat(val), []);
// [1, 2, 3, 4]

Seems like your value is split into a new array that you do not iterate over to create a 1 dimensional array. I have two suggestions. Either iterate over the split array inside the loop, or concatenate all the comma separated strings first, them do the split.

var dataArray = [];
var list = c.data.list;

list.forEAch(function(listItem){
  var values = listItem.required_fields.display_value.split(',');
  values.forEach(function(value){
    dataArray.push(value);
  });
}

c.data.required_fields = dataArray;

Or perhaps:

var values = "";
var list = c.data.list;

list.forEAch(function(listItem){
  if( !!values ) {
    values += ",";
  }
  values += listItem.required_fields.display_value;
}

c.data.required_fields = values.split(',');

To get your code working, you could simply change it to

c.data.required_fields = [];
        for(var i=0; i<c.data.list.length; i++) {
            Array.prototype.push.apply(c.data.required_fields,
                c.data.list[i].required_fields.display_value.split(',')             
            );
        }

Like that, you call Array's push method on c.data.required_fields and use the contents of c.data.list[i].required_fields.display_value.split(',') as individual arguments rather than an array that'll be pushed into another array.

You can simply merge array like.

Join array entries with ,. This will give you a single string with , as delimiter and then you can split them with , to get array

var r = [["Name","a","b"], ["Age","x","y"], ["hope","g","h"]];

console.log(r.join(",").split(','))