为什么console.log（fibonacci（3））; 打印未定义？

Question

let mem = [0, 1, 1];

const fibonacci = (n) => {
    if (n < 2) {
        return mem[n];
    } else if (n == 2) {
        console.log(mem.length-1);
        console.log((mem[mem.length-1]));
        // return (mem[mem.length - 1]);
        return (mem[mem.length-1]); // why this return statement return undefined???
    }
    else {
        mem.push(mem[mem.length - 1] + mem[mem.length - 2]);
        fibonacci(n - 1);
    }
};

console.log(fibonacci(3));

Expected output: 2..

Answer 1

Modify your code as follows:

let mem = [0, 1, 1];

const fibonacci = (n) => {
    if (n < 2) {
        return mem[n];
    } else if (n == 2) {
        console.log(mem.length-1);
        console.log((mem[mem.length-1]));
        // return (mem[mem.length - 1]);
        return (mem[mem.length-1]); // why this return statement return undefined???
    }
    else {
        mem.push(mem[mem.length - 1] + mem[mem.length - 2]);
        return fibonacci(n - 1);
    }
};

console.log(fibonacci(4));

You missed return before fibonacci(n - 1) in else .