Addition assignment did not run as expected in Javascript's recursive functions

Question

For the following functions that use addition assignment, the value of ans is always changed to 1 when an integer greater than 1 is passed

let ans = 0;
function reduce(num){
    if(num == 0){
        return 0;
    }
    ans += reduce(num-1);
    return 1;
}
reduce(5);
console.log(ans); // 1

But when I save the recursive result to another variable first, the function works fine (ans will be changed to num-1)

let ans = 0;
function reduce(num){
    if(num == 0){
        return 0;
    }
    let tmp=reduce(num-1);
    ans += tmp;
    return 1;
}
reduce(5);
console.log(ans); // 4

How does this happen? Is this a feature of Javascript or a bug in the environment?

I have tried similar C codes and they all behave the same

#include<stdio.h>
int reduce(int num);
int ans;
int main(){
    ans = 0;
    reduce(5);
    printf("%d",ans); // 4
}

int reduce(int num){
    if(num == 0){
        return 0;
    }
    int tmp=reduce(num-1);
    ans += tmp;
    // ans += reduce(num-1);
    return 1;
}

Environment:

Microsoft Edge 97.0.1072.55
JavaScript V8 9.7.106.18

JMP · Accepted Answer · 2022-01-15 09:45:26Z

3

This happens because in JavaScript ans += reduce(num-1) is evaluated as:

ans = ans + reduce(num-1)

and not as:

ans = reduce(num-1) + ans;

(See (13.15.2) Runtime semantics: evaluation in the ECMAScript specs).

So ans is evaluated before the recursive call is made, and it is that evaluated value that is used as the operand of the addition.

You can avoid such unexpected behaviour by avoiding functions with side-effects. A pure recursive solution would be:

function reduce(num) {
    if (num < 2) {
        return 0;
    }
    return reduce(num-1) + 1;
}
let ans = reduce(5);
console.log(ans); // 4

edited Jan 15, 2022 at 9:45

JMP

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answered Jan 15, 2022 at 9:16

trincot

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2 Comments

Nina Scholz Over a year ago

it's really tricky ...

ikhvjs Over a year ago

That's why never use global scope variable inside recursive function.

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