Recursion in JavaScript With IF and WHILE

Question

I just come up with little strange behavior in Javascript Recursion. My code

var temp1 = function(maxLength,accNumber) {

    if(accNumber.length < maxLength)
    {
        accNumber = '0'+accNumber; 
        temp1(maxLength,accNumber);
    }
    return accNumber;
};
console.log(temp1(5,"23"));

So Here I am expecting output as "00023" but its giving "023" which is not expected.

But same code with while loop I am getting expected output. I am not understanding why.

var temp1 = function(maxLength,accNumber) {

    while(accNumber.length < maxLength)
    {
        accNumber = '0'+accNumber; 
        temp1(maxLength,accNumber);
    }
    return accNumber;
};
console.log(temp1(5,"23"));

Which give me output "00023" as expected.

cнŝdk · Accepted Answer · 2016-08-16 14:03:02Z

2

In fact you are missing a return statement inside your if block:

var temp1 = function(maxLength,accNumber) {

    if(accNumber.length < maxLength)
    {
        accNumber = '0'+accNumber; 
        return temp1(maxLength,accNumber);
    }
    return accNumber;
};
console.log(temp1(5,"23"));

So the code wasn't returning anything if the if condition was passed, that explains why you got 023 instead of 00023.

answered Aug 16, 2016 at 14:03

cнŝdk

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Comments

Chris Wissmach · Accepted Answer · 2016-08-16 14:03:23Z

1

Change your recursive call from

temp1(maxLength,accNumber);

to

return temp1(maxLength,accNumber);

Without the new return above, your code is detecting that accNumber.length is less than maxLength and correctly adding a '0' to the front. However, your recursive call is not modifying the local variable accNumber, and not returning, so it is basically doing nothing. After the recursive call, the program will exit the if block, and return the original accNumber, which is 023.

edited Aug 16, 2016 at 14:03

answered Aug 16, 2016 at 13:59

Chris Wissmach

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3 Comments

Chris Wissmach Over a year ago

@AgataB My edit had nothing to do with the logic of my answer, just added a description of why my answer works.

AgataB Over a year ago

jsfiddle.net/qa6chtzt in this jsFiddle I get Maximum call stack size exceeded. Unless I misunderstood what you're changing.

Chris Wissmach Over a year ago

return accNumber stays the same, temp1(maxLength,accNumber); is the one that is changed.

AgataB · Accepted Answer · 2016-08-16 14:14:05Z

1

In the if loop, it's detecting that the length is smaller than maxLength, so it adds the 0 at the start and is done with the loop. It doesn't check again, if the length is still smaller. So after adding a single 0, it is done with the loop and returns output.

On the other hand the while loop will keep checking after each iteration.

Your function with if should look something like this:

function(maxLength,accNumber) {

    if(accNumber.length < maxLength)
    {
        accNumber = '0'+accNumber;
        return temp1(maxLength,accNumber);
    }
        return accNumber;

};

edited Aug 16, 2016 at 14:14

answered Aug 16, 2016 at 13:59

AgataB

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1 Comment

Benjamin Dean Over a year ago

The code continues to execute until the if statement fails, but the value of the variables while returning back up to the first iteration of the recursion are lost (except the first value, which just gets returned). See my answer (which references answers other people have given for this as well).

Community · Accepted Answer · 2017-05-23 11:51:35Z

This question appears to be a duplicate of Strange result using recursion with while loop, but I wanted to play with this to see if type coercion might be involved.

I have re-authored this here, uncomment the console.logs to see more detail: this jsfiddle.

This is a scope issue from what I can tell, the if is creating a new scope each time and upon finally exiting, returns the first recursive value for accNumber (023) rather than the innermost recursive scope value (00023). The difference is that the while loop doesn't lose scope in the same way the if version of this does.

var temp1 = function(maxLength,accNumber) {

    if(accNumber.length < maxLength) // There is no iteration
    {
        accNumber = '0' + accNumber;
        temp1(maxLength,accNumber); // This creates a new scope
    }
    return accNumber; // You're only returning the initial iteration result
};
console.log(temp1(5,"23"));

But in the while version...

var temp1 = function(maxLength,accNumber) {

    while(accNumber.length < maxLength)
    {
        accNumber = '0'+accNumber; 
        temp1(maxLength,accNumber); // Since it is in a while, scope is maintained
    }
    return accNumber;
};
console.log(temp1(5,"23"));

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