flatten-1.js

function flatten(array){
  // a place to store the flattened array
  let flattened = [];

  // iterate over the array
  for (let i = 0; i < array.length; i++) {
    const currentValue = array[i];
    // if the current value is an array
    if (Array.isArray(currentValue)) {
      for (let j = 0; j < currentValue.length; j++) {
        const innerValue = currentValue[j];
        // iterate over the elements of the array and push into our flatten array
        flattened.push(innerValue);
      }
    } else {
      // push value into array
      flattened.push(currentValue);
    }
  }
      
  // return flattened array
  return flattened;
}

function flatten(array){
  let flattened = [];
  array.forEach((currentValue) => {
    if (Array.isArray(currentValue)) {
      currentValue.forEach((innerValue) => {
        flattened.push(innerValue);
      });
    } else {
      flattened.push(currentValue);
    }
  });
  return flattened;
}

function flatten(array){
  let flattened = [];
  array.forEach((currentValue) => {
    if (Array.isArray(currentValue)) {
      flattened = [...flattened, ...currentValue];
    } else {
      flattened.push(currentValue);
    }
  });
  return flattened;
}

function flatten(array){
  let flattened = [];
  array.forEach((currentValue) => {
    if (Array.isArray(currentValue)) {
      flattened = [...flattened, ...currentValue];
    } else {
      flattened = [...flattened, currentValue];
    }
  });
  return flattened;
}

function flatten(array){
  return array.reduce((flattened, currentValue) => {
    if (Array.isArray(currentValue)) {
      return [...flattened, ...currentValue];
    }
    flattened.push(currentValue);
    return flattened;
  }, []);
}

console.log(
  flatten([]),
  []
);
console.log(
  flatten([1, 2, 3]),
  [1, 2, 3]
);
console.log(
  flatten([[1, 2, 3], ['a', 'b', 'c'], [1, 2, 3]]).toString() ===
  [1, 2, 3, 'a', 'b', 'c', 1, 2, 3].toString()
);
console.log(
  flatten([[3, 4, 5], [[9, 9, 9]], ['a,b,c']]).toString() ===
  [3, 4, 5, [9, 9, 9], 'a,b,c'].toString()
);
console.log(
  JSON.stringify(flatten([[[3], [4], [5]], [9], [9], [8], [[1, 2, 3]]])) ===
  JSON.stringify([[3], [4], [5], 9, 9, 8, [1, 2, 3]])
);

// postalrat
// if you want to keep it pretty and use a for you could do: for (const a of array)

// Ashish S
// I wonder if spread operator can be used here

// postalrat
// you could use the spread operator like: [].concat(...[[1],[2],[3]])
// and use it to solve: array => [].concat(...array)