equalSides.js

// Equal Sides of an Array from http://www.codewars.com/kata/equal-sides-of-an-array/train/javascript

/*You are going to be given an array of integers. Your job is to take that array and find an index N where the sum of the integers to the left of N is equal to the sum of the integers to the right of N. If there is no index that would make this happen, return -1.

For example:

Let's say you are given the array {1,2,3,4,3,2,1}:
Your function will return the index 3, because at the 3rd position of the array, the sum of left side of the index ({1,2,3}) and the sum of the right side of the index ({3,2,1}) both equal 6.

Let's look at another one.
You are given the array {1,100,50,-51,1,1}:
Your function will return the index 1, because at the 1st position of the array, the sum of left side of the index ({1}) and the sum of the right side of the index ({50,-51,1,1}) both equal 1.

Last one:
You are given the array {20,10,-80,10,10,15,35}
At index 0 the left side is {}
The right side is {10,-80,10,10,15,35}
They both are equal to 0 when added. (Empty arrays are equal to 0 in this problem)
Index 0 is the place where the left side and right side are equal.

Note: Please remember that in most programming/scripting languages the index of an array starts at 0.

Input:
An integer array of length 0 < arr < 1000. The numbers in the array can be any integer positive or negative.

Output:
The lowest index N where the side to the left of N is equal to the side to the right of N. If you do not find an index that fits these rules, then you will return -1.

Note:
If you are given an array with multiple answers, return the lowest correct index.
An empty array should be treated like a 0 in this problem.*//*eslint-disable*/

// const findEvenIndex = a => a.findIndex((e, i) => eval(a.slice(0, i).join`+`) === eval(a.slice(i + 1).join`+`));

// findEvenIndex=a=>a.findIndex((e,i)=>eval(a.slice(0,i).join`+`)==eval(a.slice(i+1).join`+`))

findEvenIndex=a=>a.findIndex((e,i,_,s=(s,e)=>eval(a.slice(s,e).join`+`))=>s(0,i)==s(i+1))

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

describe('findEvenIndex function', () => {
  it('should return 3', () => expect(findEvenIndex([1, 2, 3, 4, 3, 2, 1])).to.equal(3));
  it('should return 1', () => expect(findEvenIndex([1, 100, 50, -51, 1, 1])).to.equal(1));
  it('should return -1', () => expect(findEvenIndex([1, 2, 3, 4, 5, 6])).to.equal(-1));
  it('should return -1', () => expect(findEvenIndex([20, 10, 30, 10, 10, 15, 35])).to.equal(3));
  it('should return 24', () => {
    expect(findEvenIndex([-9807,4892,-6975,269,-8439,-6886,2433,-1058,4774,5419,8154,8118,6277,-8656,7121,3197,-8591,9746,-6669,8326,-2833,1827,9114,-4624,-6785,7773,-554,-220,2594,845,7544,104,-1891,7321,4034,9608,9620,-2756,-9991,9990,-7554,4518,6444,5145,-37445])).to.equal(24);
  });
});