Fibonacci - fast&slow

fib.js

import express from 'express';
const app = express()

app.get('/fibonacci/:n', (req, res) => {
    let fibonacci_slow = (n) => {
        if (n == 0) {
            return 0
        }
        else if (n <= 2) {
            return 1
        }
        return fibonacci_slow(n-1) + fibonacci_slow(n-2)
    }
    let fibonacci_fast = n => {
        if (n == 0) {
            return 0;
        } else if (n <= 2) {
            return 1;
        }
        let n1 = 1;
        let n2 = 0;
        let ni;
        for (let i = 2; i <= n; i++) {
            ni = n1 + n2;
            n2 = n1;
            n1 = ni;
        }
        return ni;
    };

    let n = req.params.n
    res.json({
        answer: fibonacci_fast(Number(n))
    })
})


app.listen(port, () => console.log(`Listening on http://localhost:${port}!`))