Flatten an array in Javascript without any inbuilt functions

As of 2020, we have ES2019 which introduced a great method called flat () to deal with nested arrays and get the flattened array. The flat() method accepts a single parameter <depth>.

The depth defines the layers in the array. By default, it`s 1.

const oneLevel = [ '🔥', ['🦁', '🍕'], '🦋' ];
console.log( oneLevel.flat() );  //depth: 1
// ['🔥', '🦁', '🍕', '🦋'];
const twoLevel = ['🔥',  [ '🦁', ['👸', '🤴'], ' 🦋' ], '🍉'];
console.log( twoLevel.flat() ); // depth: 1
['🔥', '🦁', ['👸', '🤴'], ' 🦋', '🍉']
console.log( twoLevel.flat(2) );  // depth: 2
// ['🔥', '🦁', '👸', '🤴', ' 🦋', '🍉']

Infinite Nested Arrays using flat()

const infiniteDeep = [...[], [..[..[[]..]..]..]]
infiniteDeep.flat(Infinity);

But, flat is a super new feature in the world of Javascript so all browser support is not there. Therefore, we calculate this using raw way

Infinite Nested Arrays without flat()

function flatArray(arr) {
   let result = [];  // In this we`ll save the output;
   let main = arr, first;
   while(main.length > 0) {   //1. Looping it
      first = main[0];
      if(Array.isArray(first)) { 
      Array.prototype.splice.apply(main, [0, 1].concat(first));
   } else {
      result.push(first);
      main.splice(0, 1);
   }
}
   return result;
}
console.log(flatArray(arr)); // Flatten array [.........] 

There you go. A simple yet useful function in the world of javascript. Hope this was helpful.