multiplying in while loop with prefix increment

Question

I'm a newbie in Javascript and I need to learn how this simple code that I just saw somewhere in the internet works:

var f = 1;
var i = 2;
while(++i<5) {
 f*=i;
}
console.log(f);

Can anybody help me understand how this loop works?

• Hector Barbossa: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Answer 1

Check comments to understand it:

var f = 1;
var i = 2;
while(++i<5) { //will increment first and then check if the incremented value is less than 5
 f*=i; //can also be written as f = f*i
}
console.log(f);

First iteration:: while (3<5) it will make f = 1*3 which is 3

Second iteration:: while (4<5) it will make f = 3*4 which is 12

Third iteration:: while (5<5) which is false so loop will stop

• NehaJ: My pleasure :) You can mark the answer as correct in case it clarified your doubt.

Answer 2

A less cryptic while loop that does the same thing would be:

var f = 1;
var i = 2;
++i; // increment i
while (i < 5) { // loop while i is less than 5
 f = f * i; // assign f * i to f. aka "scale f by i"
 ++i; // increment i
}
console.log("i:", i, "f:", f);

Here's a table

Iteration | i | f
------------------------------
Before loop | 3 | 1
After 1st | 4 | 3
After 2nd | 5 | 12

And it exits after the 2nd iteration because 5 is not less than 5.

Answer 3

The while loop is executed every time when, the expression within the braces is truthy (https://developer.mozilla.org/en-US/docs/Glossary/Truthy). Hence when i is less then 5, an iteration is executed. But ++ before i might be tricky. Prefix is added before the execution, hence, this loop will be iterated with i=2+1=3 and i=3+1=4.