array_expert_flatten_simple_impl.js

function flatten(array) {
  return array.reduce((previousValue, currentValue) => {
    if (Array.isArray(currentValue)) {
      // current value is an array, merge values and return the result
      return [...previousValue, ...currentValue];
    }

    // current value is not an array, just add it to the end of the accumulation
    return [...previousValue, currentValue];
  }, []);
}

const elements = [1, [2], 3, [4, 5], [6]];
const nextElements = flatten(elements); // [1, 2, 3, 4, 5, 6]