Cannot read property *0* of null

Answer 1

getElementById returns null if there is no match in the document. (Which then leads to exactly your error message).

This means that either you have a typo in your selector or the html or the js gets executed before your element is included into the dom.

Also, getElementById returns a single element and not an array (Node List to be precise), so the correct call would be:

document.getElementById('td1').style.color = 'blue';

Third problem:

setInterval(test(),1000);
//              /\
// this will immeditately execute the function and
// hands over the *return value* to setInterval

will not work as intended, it needs to be

setInterval(test,1000);
//            /\
// hand over the *function reference* to be executed after 1 second