Find pivot index

pivot.js

// Return the "pivot index" of the given array of numbers. The pivot index is
// the index where the sum of the numbers on the left is equal to the sum of
// the numbers on the right.
function pivot(numbers) {
	validateInput(numbers);

	// Find a pivot index by testing each index
	for (var i = 0; i < numbers.length; i++) {
		var leftSum = sum(numbers.slice(0, i));
		var rightSum = sum(numbers.slice(i + 1));

		if (leftSum === rightSum) {
			return i;
		}
	}

	// No pivot found
	return -1;
}

// More efficient, less readable version of pivot()
function pivotEfficient(numbers) {
	validateInput(numbers);

	// Keep track of running sums as we iterate array
	var leftSum = 0, rightSum = sum(numbers);

	for (var i = 0; i < numbers.length; i++) {
		rightSum -= numbers[i];

		if (leftSum === rightSum) {
			return i;
		}

		leftSum += numbers[i];
	}

	// No pivot found
	return -1;
}

// Check validity of pivot() input and throw helpful error if not valid
function validateInput(numbers) {
	// Must be array
	if ( ! Array.isArray(numbers)) {
		throw new Error("Argument must be an array");
	}

	// Must be all numbers
	if ( ! numbers.every(isNumber)) {
		throw new Error("Every array element must be a number");
	}
}

// Return the sum of the numbers in the given array
function sum(numbers) {
	return numbers.reduce(function(acc, current) {
		return acc + current;
	}, 0);
}

// Determine if the given input is a number
function isNumber(number) {
	return typeof number === "number";
}

// Test pivot() implementation
function test() {
	var assert = require("assert");

	// Bad input
	[
		undefined,
		null,
		0,
		10,
		-10,
		Infinity,
		"foo",
		[1, "foo", 2]
	].forEach(function(arg) {
		var thrown = false;
		try {
			pivot(arg);
		}
		catch (e) {
			thrown = true;
		}
		assert(thrown);
	});

	// Reference case
	assert(pivot([1, 4, 6, 3, 2]) === 2);

	// No valid pivot
	assert(pivot([]) === -1);
	assert(pivot([1, 2, 3]) === -1);

	// Multiple pivots
	assert(pivot([1, 2, 0, 0, 3]) === 2);

	// Other cases
	assert(pivot([1]) === 0);
	assert(pivot([1, 0]) === 0);
	assert(pivot([0, 1]) === 1);
	assert(pivot([1, 1, 1]) === 1);
	assert(pivot([8, 1, 5, 4, 3, 2]) === 2);

	// Negatives numbers
	assert(pivot([5, -1, 100, 6, -2]) === 2);
	assert(pivot([-8, 5, -3, -5]) === 1);
}