nested2flat.js

var test = require( "tape" );

// main function to flat nested array
const flatten = ( arr ) => {
  return arr.reduce( ( flat, toFlatten ) => {
    return flat.concat(
      Array.isArray( toFlatten ) ? flatten( toFlatten ): toFlatten
    );
  }, []);
};

// test main function
test( 'Flatten', t => {
 t.plan( 1 );
 let nestedArr = [[1,2,[3]],4];
 let expected = [1, 2, 3, 4];
 let actual = flatten( nestedArr );
 t.equals( actual.length, expected.length, 'flats nested array' );
});