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

function fold_parts(str, sep, state, fun) {
    for (const part of str.split(sep)) {
        state = fun(state, part)
    }
    return state
}

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