productOfAllOtherNumbers.js

function getProductsOfAllIntsExceptAtIndex(intArray) {
    // Make a list of the products
    if (intArray.length < 2) {
        throw new Error('Minimum two numbers needed for a product');
    }
    let productOfAllOthers = [];
    let currentProduct = 1;
    for (let i = 0; i < intArray.length; i++) {
        productOfAllOthers[i] = currentProduct;
        currentProduct *= intArray[i];
    }

    currentProduct = 1;

    for (let j = intArray.length - 1; j >= 0; j--) {
        productOfAllOthers[j] *= currentProduct;
        currentProduct *= intArray[j];
    }

    return productOfAllOthers;
}

//O(n) time and O(n) space

// Tests

let desc = 'short array';
let actual = getProductsOfAllIntsExceptAtIndex([1, 2, 3]);
let expected = [6, 3, 2];
assertArrayEquals(actual, expected, desc);

desc = 'longer array';
actual = getProductsOfAllIntsExceptAtIndex([8, 2, 4, 3, 1, 5]);
expected = [120, 480, 240, 320, 960, 192];
assertArrayEquals(actual, expected, desc);

desc = 'array has one zero';
actual = getProductsOfAllIntsExceptAtIndex([6, 2, 0, 3]);
expected = [0, 0, 36, 0];
assertArrayEquals(actual, expected, desc);

desc = 'array has two zeros';
actual = getProductsOfAllIntsExceptAtIndex([4, 0, 9, 1, 0]);
expected = [0, 0, 0, 0, 0];
assertArrayEquals(actual, expected, desc);

desc = 'one negative number';
actual = getProductsOfAllIntsExceptAtIndex([-3, 8, 4]);
expected = [32, -12, -24];
assertArrayEquals(actual, expected, desc);

desc = 'all negative numbers';
actual = getProductsOfAllIntsExceptAtIndex([-7, -1, -4, -2]);
expected = [-8, -56, -14, -28];
assertArrayEquals(actual, expected, desc);

desc = 'error with empty array';
const emptyArray = () => (getProductsOfAllIntsExceptAtIndex([]));
assertThrowsError(emptyArray, desc);

desc = 'error with one number';
const oneNumber = () => (getProductsOfAllIntsExceptAtIndex([1]));
assertThrowsError(oneNumber, desc);

function assertArrayEquals(a, b, desc) {
    const arrayA = JSON.stringify(a);
    const arrayB = JSON.stringify(b);
    if (arrayA !== arrayB) {
        console.log(`${desc} ... FAIL: ${arrayA} != ${arrayB}`)
    } else {
        console.log(`${desc} ... PASS`);
    }
}

function assertThrowsError(func, desc) {
    try {
        func();
        console.log(`${desc} ... FAIL`);
    } catch (e) {
        console.log(`${desc} ... PASS`);
    }
}