recursion in javascript

Question

cant really wrap my head around it. So:

var power = function(base, exponent) {  
    if (exponent === 0) {
        return 1;
    } else {
        return base * power(base, exponent - 1);
    }
};

I guess it works this way:

base = 2
exponent = 5
exponent !== 0 so base case is skipped
return 2 * power(2, 4)
exponent = 4
return 2 * power(2, 3)
exponent = 3
return 2 * power(2, 2)
exponent = 2
return 2 * power(2, 1)
exponent = 1
return 2 * power(2, 0)
exponent = 0
return 1

but when return 2* power(2,4) is running should not power * 2 and then * 4 as it is in between brackets?

Answer 1 · 2014-05-06T13:37:54Z

May 6, 2014 1:37pm

To expand upon what Ryan Gordon said and to help you understand better what is going on with this function, you can copy the below code, open your browser Dev Tools' console, execute the code and see what is happening.

var result = 0;
var base = 2;
var exponent = 5;

var power = function(base, exponent) {  
    var r = 0;

    // output the value of exponent
    console.log("exponent: " + exponent);

    if ((exponent === 0) || (typeof exponent !== "number")) {
        console.info(exponent); // this should output 0;
        r = 1;
    } else {
        r = base * power(base, exponent - 1);
    }

    // output the value of r
    console.log("r: " + r);
    return r;
};

result = power(base, exponent);

// output the final value (result)
console.log("result: " + result);

The above should output the following:

"exponent: 5"
"exponent: 4"
"exponent: 3"
"exponent: 2"
"exponent: 1"
"exponent: 0"
: 0
"r: 1"
"r: 2"
"r: 4"
"r: 8"
"r: 16"
"r: 32"
"result: 32"

Answer 2 · 2014-05-06T12:55:59Z

May 6, 2014 12:55pm

It looks like you're not finishing the recursion. You've gone through all the if statements but the function calls are not complete until after 1 is returned.

After the return 1 happens the function call power(2,1) can be finished returning 2, then power(2,2) returning 4, then power(2,3) returns 8, then power(2,4) returns 16, then power(2,5) can return the final answer 32.

Answer 3 · 2014-05-06T13:00:35Z

May 6, 2014 1:00pm

but how the 2 * power(2, 4) works for example. I thought it should : 2* 2 // 2 * base 2* 4 // 2* exponent .

how come its not multiplying exponent?

Answer 4 · 2014-05-06T13:32:15Z

May 6, 2014 1:32pm

I understand that bit. but i dont understand why when we calling return base * power(base, exponent - 1);

it only multiply base * base but not multiplying base time exponent witch is next to base after the come between the brackets.

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Ireneusz Kopta

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recursion in javascript

4 Answers

Sean T. Unwin

Sean T. Unwin

Ryan Gordon

Ryan Gordon

Ireneusz Kopta

Ireneusz Kopta

Ryan Gordon

Ryan Gordon

Ireneusz Kopta

Ireneusz Kopta

Ryan Gordon

Ryan Gordon