javascript loop

(Approaching your question as a general question about loop constructs and how they are 'typically' used)

It depends on what your exit condition is...

• Use a for loop if you want to iterate a fixed number of times and need to use an incrementing (or decrementing) value in your loop
• Use a while loop if your exit condition can be expressed as a boolean statement, and the minumum number of iterations of your loop is zero (because the boolean statement is located before the loop code)
• Use a do...while loop if your exit condition can be expressed as a boolean statement, and the minumum number of iterations of your loop is one (because the boolean statement is located after the loop code)

I think you want to perform the division part with loop though what you have done is more efficient.

I hope this is not a homework :)

Here is the answer:

 var loan = 1000000;
 var amoyear = 6000;
 var answer;
 for(answer=1; amoyear*answer<=loan; answer++);

The above code uses simple for loop and I think it would be the best for this situation.

A rough, generic answer to your question (not considering your particular use-case):

If you want to loop over an interval, increasing/decreasing a value by a fixed amount in each step, then use a for-loop since it is very idiomatic for that (in almost any imperative or object-oriented language).

For almost all other purposes, use the while-loop.

I'd say that do-while is not very useful and not seen often, even if you of course can find situations for that too. But some slight refactoring can often change it into a while-loop that is (usually) more readable anyway.

Just taking a superficial look at your unformatted code.. it looks like you want to do a loop to calculcate the amount of time different loans will take to pay off. When your working with numbers, it's usually easier to work with a for loop. The following examples both calculate time to pay off loans between $100,000 and $20,000,000 (at intervals of $100,000)

var maxLoanAmount=20000000;
var amoyear = 6000; 
for (var loan = 100000; loan<=maxLoanAmount; loan = loan + 100000){

    var answer = loan / amoyear; 
    document.write("A loan of " + loan  + " SEK is paid after " + Math.ceil(answer) + " years if installment is " + (amoyear/12) + " kr / month. ");
}

although you can easily do the exact same thing with a while loop.

var maxLoanAmount=20000000;
var amoyear = 6000; 
var loan = 100000;
while (loan<=maxLoanAmount){
    var answer = loan / amoyear; 
    document.write("A loan of " + loan  + " SEK is paid after " + Math.ceil(answer) + " years if installment is " + (amoyear/12) + " kr / month. ");
    loan = loan + 100000;
}

Use for loop when you now the number of items before looping it. For example looping via elements of an array or array-like object:

var items = [];
....
for(var i = 0; i < items.length; i++){
...
}

If you iterate via fields of some object you may use the following form of for loop:

var obj = {};
...
for(var field in object){
 //access each value as object.field
}

while-loop, is used, when you don't know the term when loop is finished.

do-while loop use also when you don't know the term when loop is finished but you need to iterate at least one time Compare:

do{
 //do something, then check the term
}while(check the term)

vs.

while(check the term){
 //do something 
}

PS: there is a form of for loop - so called foreach, but i'm not sure it is standard, so I don't use it yet.