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 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"];

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

1var arr1 = [1, 2, [3, 4]];
2arr1.flat(); 
3// [1, 2, 3, 4]
4
5var arr2 = [1, 2, [3, 4, [5, 6]]];
6arr2.flat();
7// [1, 2, 3, 4, [5, 6]]
8
9var arr3 = [1, 2, [3, 4, [5, 6]]];
10arr3.flat(2);
11// [1, 2, 3, 4, 5, 6]

1const arrays = [
2      ["$6"],
3      ["$12"],
4      ["$25"],
5      ["$25"],
6      ["$18"],
7      ["$22"],
8      ["$10"]
9    ];
10const merge3 = arrays.flat(1); //The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
11console.log(merge3);
12