How do I pass a parameter to a setTimeout() callback?

Question

What is the difference between this:

function blankWord(){
    console.log('blank!');
    setTimeout(blankWord, 5000);
}
blankWord();

Which calls the function every 5 seconds as it should, and this:

function blankWord(t){
    console.log('blank!');
    setTimeout(blankWord, t);
}
blankWord(5000);

Which calls the function repeatedly insanely fast?

Answer 1

Since you are missing the parameter in the second form you pass undefined from the second call on, which will essentially result in a timeout of 4ms (which is the browser minimum).

Use a function wrapper, to safely pass the parameters you need:

function blankWord(t){
    console.log('blank!');
    setTimeout(function(){blankWord(t)},t);
}
blankWord(5000);

Passing the parameters as third parameters knocks out older IEs, that's why you should not use this until IE8 is dead.

Answer 2

The first script calls setTimeout with a second argument of 5000 every time.

The second script calls it with t. The first time that is 5000 (from blankWord(5000);). Each subsequent time it is undefined (from setTimeout(blankWord).

If you want to pass arguments, do so by passing them in an array as the third argument of setTimeout.

setTimeout(blankWord, t, [t])

See mdn for a polyfill to support older browsers that don't recognise the three argument form of the function.