Introduction to For Loops

1. Introduction to For Loops See Beginning JavaScript (Paul Wilton) p. 87

2. For Loop Example

3. Result in browser

4. var SomeHTML=""; for(i=1; i<=100;i++) { SomeHTML+="All work and no play makes Jack a dull boy. "; } document.write(SomeHTML); for keyword that indicates a for loop repetitive structure i=1 where we start counting the iterations i<=100 condition under which we will continue to count i++ the incrementing — this does the counting, the changing of the counting variable, in this case by 1. It could also be written i=i+1 or i+=1 The parts above are in parentheses and separated by semicolons. Then comes a set a curly brackets. The line(s) of code in the curly brackets constitute a single “iteration.”

5. Variation There are 3 loops this time; the middle one changes the iteration code (inside the curly brackets) somewhat.

6. Variation Result

7. Second Variation The counting variable is concatenated with the text – so that the counting is seen on the page.

8. Second Variation Result

9. Third Variation That’s two minus signs in a row. A change in the initialization (starting point) to start at 10, a change in the condition appropriate to counting down, a change from incrementing to decrementing.

10. Third Variation Result