# Web frontend exercises¶

## JavaScript: Clock¶

This is a real, former exercise from the course "Computing Applications" (Tietotekniikka sovelluksissa).

### Exercise instructions¶

Open the exercise template clock.html (exercises/clock/clock.html) and implement the following JavaScript functionality.

1. First, create a JavaScript function which changes the text in the H1 heading in the template to "Hello World".

2. Then modify the function so that the heading has current time in the format hh:mm:ss.

You can access the time and date by the JavaScript object Date(). You can, for example, read the hours as an integer in the following way:

var pvm = new Date();
var hours = pvm.getHours();


You can read minutes and seconds by methods getMinutes and getSeconds(), respectively.

NOTE! At this point, the clock does not need to work correctly with numbers less than ten. (See part 4.)

Hint. Unlike Python, in JavaScript one can concatenate a string with a string and a string with an integer. The integer is automatically converted into a string and the result is a string. Example:

"ABC" + "1" + 23 ==> "ABC123"

1. Modify your code so that the clock updates in 100 millisecond intervals.

Calling a function repeatedly

If you replace the function call func_name() with function setInterval(func_name, time), JavaScript will begin to call the function func_name() repeatedly with intervals. The time is in milliseconds.

You will notice that the clock does not still work quite right. If a number is less than ten, the leading zero is not shown, because the times are integers.

1. Write a function which adds a leading zero to a number if the number is less than ten. Use this function to correct the clock.

2. Modify the clock function so that the text in the H1 element has color red on even seconds and color blue on odd seconds.

Hint: a % b computes a division remainder. See also: arithmetic operations in JavaScript.

A+ presents the exercise submission form here.

### Model solution¶

The model solution fto this exercise is in the aplus-manual directory exercises/clock/model_answer.html.