const str = "The quick brown fox jumps over the lazy dog.\n".repeat(100)

function fold_parts(str, sep, state, fun) {
    let position = 0
    while (true) {
        let idx = str.indexOf(sep, position)
        if (idx < 0) {
            return fun(state, str.slice(position))
        }

        state = fun(state, str.slice(position, idx))
        position = idx + sep.length
    }
}

let sum = 0
for(let i = 0; i < 1000000; ++i) {
    sum += fold_parts(str, '\n', 0, (acc, line) => acc + line.length)
}
console.log(sum)