CODEWITHSUNDEEP
javascriptfunctionrecursionundefined
Ron
Ron

Reputation: 41

Why this recursive javascript function returns undefined?

In the following recursive function, I expect the function to return "arrived" at the end but instead, it returns undefined. Isn't that when the excution goes in the if block the code should return? Appreciate your comments on this.

function myFun(i){
    if(i===0){return ('arrived');}
    i = i - 1;
    myFun(i);
}

If I change the code as follows then it'll return "arrived" but still don't know why the above doesn't return "arrived".

function myFun(i){
    if(i===0){return ('arrived');}
    i = i - 1;
    return myFun(i);
}

Upvotes: 3

Views: 216

Answers (2)

Mulan
Mulan

Reputation: 135197

Recursion is a functional heritage and so writing your program in a functional style will yield the best results.

This means avoiding things like

  • imperative style statements for, if, switch, etc that do not return a value
  • mutation or variable reassignments like i = i + 1

const myFun = i =>
  i === 0
    ? "arrived"
    : myFun (i - 1)
    
console.log (myFun (10))
// "arrived"

Notice expressions, unlike statements, evaluate to a value. We made the following changes

  • function statement function myFun (i) { ... } replaced with a function expression myFun (i) => ...
  • if statement replaced with ternary expression, eg condition ? ifTrue : ifFalse
  • variable assignment statement i = i - 1; myFun(i) replaced with expression myFun(i - 1)

Note, the return statement itself is a side effect and has little use in a functional JavaScript program.

Going along with the other answer here, all code paths must return a value! An advantage to writing your program in a functional style means you can't write a function that doesn't return a value, nor can you write a conditional with only one branch.

TL;DR: use functional style and your problem automatically vanishes

Upvotes: 1

Peter B
Peter B

Reputation: 24137

The first function does not return a value because all code paths must return a value. And after the first line, there is no return statement. It only returns a value when called with 0 as parameter.

Upvotes: 7

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