Why does this javascript function keep returning an array as undefined?

Question

I have this simple function "decode" that takes in 2 arrays as input, where the second array is used to decode the first array.

The starting input (not when the function recurses) must always be of the following format:

1st array is of length 1
2nd array is of a length that is a power of 2, minus 1 (1,3,7,15,...)

Example input: ([4],[0,2,6])

For some reason, my code always returns undefined when I try to return a decoded array. In fact, I can't seem to return anything other than undefined, even when I change the return statement to something like "return false". The log statements show that the correct values are being captured for both arrays, leaving me very confused.

Here's my code:

var decode = function(A, B){
    console.log("A: "+A+" B:"+B);
    console.log(B.length);
    if(B.length===0){
        return A;
    }
    var newA = [];
    var newB = [];
    var act = 0;
    for(let i=0; i<A.length; i++){
        newA[act] = A[i] - (B[i]/2);
        newA[act+1] = A[i] + (B[i]/2);
        act+=2;
        newB = B.slice(i+1);
    }
    decode(newA, newB);
}

console.log("Answer is" + decode([4], [0,2,6]));

This will always return undefined, regardless of what you make the return statement. Console.log(A); on the other hand is giving me the correct value for what I want to return.

Thank you very much for the help! Greatly appreciated.

@dwjohnston there is one... always returning A... But indeed some sort of return is needed close to the end... making this post simple "typographical error". — Alexei Levenkov
– Alexei Levenkov, Commented Feb 11, 2019 at 5:03
@smnbbrv Thanks! This gave me what I wanted. However, I still don't understand why this worked and not what I did... I'm a little confused maybe about recursion/return statements. — Anon User
– Anon User, Commented Feb 11, 2019 at 5:10

Wai Ha Lee · Accepted Answer · 2019-02-11 05:33:10Z

1

The problem is that if B.length != 0, there is no return value. Change

decode(newA, newB);

to

return decode(newA, newB);

edited Feb 11, 2019 at 5:33

Wai Ha Lee

8,856102 gold badges61 silver badges100 bronze badges

answered Feb 11, 2019 at 5:13

Robin Ding

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