pivot.js

function pivot (array) {
    let total = array.reduce((accumulator, value) => accumulator + value, 0)
    let left = 0

    // Iterate over each item in array
    for (let i = 0; i < array.length; i++) {
        // Subtract its value from the total
        total -= array[i]
        // If current total is equal to the left total, then
        // we've hit the index we need, returns the index
        if (total === left) return i
        // Otherwise we carry on and increment the left total
        left += array[i]
    }
    // No such index exists
    return -1
}

// Barebones unit testing
function check (input, expected) {
    if (pivot(input) === expected) return true
    return false
}

[
    {arr: [1,4,6,3,2], exp: 2},
    {arr: [1,4,1,5,3,3], exp: 3},
    {arr: [8,1,3,4,1], exp: 1},
    {arr: [2,4,4,2,9], exp: -1}
].forEach(test => {
    console.log(`${test.arr} should return ${test.exp}: ${check(test.arr, test.exp)}`)
})