Getting return undefined on recursive function javascript

Question

I heard recursive function is powerful so instead going through loop i tried to create a function which increments a number until it reach certain points. when it reaches i trie

Answer 1

Well you could just use the return statement and you can check the code in the console like

var i=1;
function rec(){
i++;
console.log(i);
if(i > 100){
    return i;
}else{
    return rec();
}
}

console.log(rec());

Answer 2

rec will either return i (if i is over 100) or undefined (otherwise).

When you call it here:

console.log(rec());

i will be 1 so it will return undefined.

You never do anything with the return value when the return value is over 100.

You need to return the result of the recursive call:

} else {
    return rec();
}

so when it is over 100, the value gets passed back up the stack.

it this kind of recursion is good then for loop?

No. It is horribly inefficient.

Answer 3

Try putting the following code after your function ends:

typeof rec === 'undefined'

Answer 4

The comment by vaultah is correct:

var i=1;
function rec(){
    i++;
    console.log(i);
    if(i > 100){
        return i;
    }else{
        return rec();
    }
}

snippet.log(rec());

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

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Let's take an example that only counts to > 5 and add some output so you can see what happens more easily (but again, stepping through the code with a debugger is the right way to see how this works):

var indent = "";
var i=1;
function rec(){
    var rv;
    i++;
    indent += " ";
    if(i > 5){
      snippet.logHTML("<p>" + indent + i + " > 5, returning " + i + "</p>");
        rv = i;
    }else{
        snippet.logHTML("<p>" + indent + i + " is not > 5, calling rec</p>");
        rv = rec();
        snippet.logHTML("<p>" + indent + "Got " + rv + " back from rec, returning it</p>");
    }
    indent = indent.substring(0, indent.length - 6);
    return rv;
}

snippet.logHTML("<p>Final result: " + rec() + "</p>");

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

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