1// flat(depth), 
2// depth is optional: how deep a nested array structure 
3//		should be flattened.
4//		default value of depth is 1 
5
6const arr1 = [1, 2, [3, 4]];
7arr1.flat(); 
8// [1, 2, 3, 4]
9
10const arr2 = [1, 2, [3, 4, [5, 6]]];
11arr2.flat();
12// [1, 2, 3, 4, [5, 6]]
13
14const arr3 = [1, 2, [3, 4, [5, 6]]];
15arr3.flat(2);
16// [1, 2, 3, 4, 5, 6]
17
18const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
19arr4.flat(Infinity);
20// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

1var arrays = [
2  ["$6"],
3  ["$12"],
4  ["$25"],
5  ["$25"],
6  ["$18"],
7  ["$22"],
8  ["$10"]
9];
10var merged = [].concat.apply([], arrays);
11
12console.log(merged);

1var multiDimensionArray = [["a"],["b","c"],["d"]]; //array of arrays
2var flatArray = Array.prototype.concat.apply([], multiDimensionArray); //flatten array of arrays
3console.log(flatArray); // [ "a","b","c","d"];

1// Although this now may be an older version of how to faltten an 
2// array of arrays. I still want to post it so some may have an understanding 
3// of how it works
4
5function falltenArray(arr) {
6  
7  let result = [...arr];
8  let flattened = false;
9  let counter = 0;
10  
11  while (flattened === false){
12	// checks to see if the element at the counter index is an array
13      if (Array.isArray(result[counter])){
14        // unpacks the current array element back into the array
15        result.splice(counter, 1, ...result[counter]);
16        // if so the counter should start at the beginning of the array
17        counter = 0;
18        
19      } else {
20        counter += 1;
21      }
22
23      if (counter === result.length){
24        flattened = true;
25      }
26  }
27  
28  return result;
29}

1const arr = [1, 2, [3, 4]];
2
3// To flat single level array
4arr.flat();
5// is equivalent to
6arr.reduce((acc, val) => acc.concat(val), []);
7// [1, 2, 3, 4]
8
9// or with decomposition syntax
10const flattened = arr => [].concat(...arr);
11

1function flatten(arr) {
2  return arr.reduce(function (flat, toFlatten) {
3    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
4  }, []);
5}
6