While loop and increment in JavaScript

Question

In the following function I need to increment the years variable in order to find the amount of years that need to pass before I reach a desired profit. I noticed that this function does not work if I use years++ instead of ++years. I understand the difference between the two methods of increment, however, I still do not understand while in this particular case years++ causes the loop to be executed only once.

 function calculateYears(investment, interestRate, tax, desiredProfit) {
        var years = 0;
        while(investment < desiredProfit && ++years){
          investment += (investment * interestRate) * (1 - tax);
        }
    
        return years;
    }
    var years = calculateYears(1000, 0.05, 0.18, 1100);
    console.log(years);

alessandrio · Accepted Answer · 2018-01-17 13:31:46Z

4

I still do not understand while in this particular case years++ causes the loop to be executed only once.

because && years++ translates to && 0 which will translates to falsey value.

If you want to use years++, initialize years to 1

function calculateYears(investment, interestRate, tax, desiredProfit) {
    var years = 1;
    while(investment < desiredProfit && years++){
      investment += (investment * interestRate) * (1 - tax);
    }

    return years - 1;
}
console.log(calculateYears(1000, 0.05, 0.18, 1100));

edited Jan 17, 2018 at 13:31

alessandrio

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answered Jan 17, 2018 at 11:31

gurvinder372

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7 Comments

Max Maximilian Over a year ago

It returns 4 instead of 3. See my answer

gurvinder372 Over a year ago

@MaxMaximilian Thanks for pointing out, there will be an extra increment while checking for final while, I have made the updates.

bakrall Over a year ago

@gurvinder372 your explanation is very clear and useful. Thank you! I made a test when changing var years = 1. It gave interesting result - while loop works, which is perfect. But it does not give the logic needed - as the value of investment in the first loop need to be taken as the one before the first year passed. In conclusion, for the function to give proper result I need to initialize it with 0. But having your explanation, as well as answers from others, I can understand now why the things work the way they work. Thanks a lot!

Max Maximilian Over a year ago

I think, that way isn't very clean. That works, which is good, but 1) it makes unnecessary calculations after the loop, 2) on each loop it checks && years++ which makes unnecessary calcs too. As programmers we should make code cleaner, because that can show up in bigger projects where you need to create something which does perform better in speed etc. But yeah, that explains the root of the issue, so good answer gurvinder372

gurvinder372 Over a year ago

@MaxMaximilian I agree that this isn't the cleanest and there can be more readable ways of doing this.

|

techfoobar · Accepted Answer · 2018-01-17 11:31:35Z

4

It gets executed only once because the value of years used for checking truthiness is 0, i.e. before incrementing.

MDN Documentation

answered Jan 17, 2018 at 11:31

techfoobar

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Comments

Max Maximilian · Accepted Answer · 2018-01-17 11:34:40Z

2

You should use years++ inside the loop:

function calculateYears(investment, interestRate, tax, desiredProfit) {
    var years = 0;
    while(investment < desiredProfit){
      investment += (investment * interestRate) * (1 - tax);
      years++;
    }

    return years;
}
alert(calculateYears(1000, 0.05, 0.18, 1100));

JSFIDDLE

answered Jan 17, 2018 at 11:34

Max Maximilian

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Comments

SF..MJ · Accepted Answer · 2018-01-17 11:34:54Z

0

while(investment < desiredProfit && ++years)

this condition will not execute because as years is 0 and it increment after the 1 loop so it is like

desiredProfit && 0 -> false/0

and after

investment < 0 //is always false

answered Jan 17, 2018 at 11:34

SF..MJ

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Comments

Nazehs · Accepted Answer · 2018-01-17 11:47:53Z

0

You did forget that your while loop is not incrementing the value of years

 while(investment < desiredProfit && ++years){
  investment += (investment * interestRate) * (1 - tax); years++;
}

answered Jan 17, 2018 at 11:47

Nazehs

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Comments

Tae Soo Kim · Accepted Answer · 2020-12-12 00:02:59Z

0

Why not do something like this

let i = -6;
let end = 9;
while ((i++ || true) && i < end) {
...do something...
}

edited Dec 12, 2020 at 0:02

answered Dec 11, 2020 at 16:23

Tae Soo Kim

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Comments

hain shaker · Accepted Answer · 2018-01-17 13:35:15Z

-1

Because && years++ translates to && 0 which will translates to false value.

If you want to use years++, initialize years to 1.

edited Jan 17, 2018 at 13:35

user1773603

answered Jan 17, 2018 at 12:09

hain shaker

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