Flatten a nested array in javascript

array-flatten.js

/* This function will flatten an array of arbitrarily nested
arrays of integers into a flat array of integers
it returns an array of flattened items

SETUP INSTRUCTIONS
 make sure you have nodejs >= V6.0 installed
 in your terminal make a new folder with mkdir flatten
 navigate to the newly created folder with cd flatten
 make a new file with touch index.js
 install mocha and chai using npm install mocha chai 
 copy the contents from this file to index.js
 run the test with ./node_modules/.bin/mocha index.js

*/


const { expect } = require('chai');

function flatten(input) {
  if (!(input instanceof Array)) {
    throw new Error('input must be an array');
  }
  const flat = input.reduce((acc, item) => {
    if (Array.isArray(item)) {
      acc.push(...flatten(item));
    } else {
      acc.push(item);
    }
    return acc;
  }, []);
  return flat;
} 

describe('flatten array', () => {
  it('should throw an error if item is not an array', () => {
    try {
      flatten('abcdef');
    } catch (error) {
      expect(error.message).to.eql('input must be an array');
    }
  });
  it('should return an empty array if the array passed in is empty', () => {
    const result = flatten([]);
    expect(result).to.eql([]);
  });
  it('should return the input array if it alreay flat', () => {
    const array = [1, 2, 3, 4];
    const result = flatten(array);
    expect(result).to.eql([1, 2, 3, 4]);
  });
  it('should flatten a nested array', () => {
    const array = [1, 2, 3, [4, 5, 6, [7, [8, [[9, [[[10]]]]]]]]];
    const result = flatten(array);
    expect(result).to.eql([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
  });
});

Hey Amos,
your code looks good! I've just one question: could you try to refactor so that const flat = gets the result of input.map?

Something like this:

  const flat = input.map((item) => {
    ...
  });

Thanks!
Stefano

Hi Stefano Thank you for your review, just seeing this, i don't think a refactor using map would work, i could use a reduce instead

function flatten(input) {
  if (!(input instanceof Array)) {
    throw new Error('input must be an array');
  }
  const flat = input.reduce((acc, item) => {
    if (Array.isArray(item)) {
      acc.push(...flatten(item));
    } else {
      acc.push(item);
    }
    return acc;
  }, []);
  return flat;
}