Array Flatten

flatten-test.js

// This test takes an array, loops through each top level index
// and logs an error if the index contains an array.
// Passes are tallied, and passes/fails are output at the end of the test.

const flattenTest = arr => {

    // Define error message and initialize pass counter
    const errorMessage = ind => `Index ${ind} contains an array`;
    let passCount = 0;
    
    arr.forEach((item, ind) => {

        // If the item is an array, log the message
        if(Array.isArray(item)){

            console.error(errorMessage(ind));
        
        // If not an array, increment pass counter
        } else {

            passCount++;

        }

    });

    // Log out results
    console.log(`${arr.length} total top level indexes.`);
    console.log(`${passCount} tests passed.`);
    console.log(`${arr.length - passCount} tests failed.`);

}

module.exports = flattenTest;

flatten.js

const flatten = arr => {

    let outputArr = [];

    // Loop through each index of the array
    arr.forEach(item => {

        // If the current item is not an array, push its value immediately.
        if(!Array.isArray(item)){

            outputArr.push(item);

        // If the current item is an array, recurse into it.
        // It's usually better to not use recursion, but in this case,
        // depth is unknown, and recursion makes sense.
        } else {

            // Spread the results of the recursion into the output array.
            outputArr.push(...flatten(item));
        
        }

    });

    return outputArr;

}

// Created for use with Node or other CommonJS
module.exports = flatten;

index.js

// Run this file using `node index.js` to see results.
const flattenTest = require('./flatten-test.js');
const flatten = require('./flatten.js');

const arr = [[1,2,[3]],4];

// First, test flatten function
console.log('Running example array with flatten function');

flattenTest(flatten(arr));

// Second, show test failures using input array
console.log('\nRunning example array without flatten function')

flattenTest(arr);