javascript - function as argument

Question

I was just wondering;

Why does this work;

 test = function(message) {
 console.log('xxx');
 }
 setTimeout(test, 3000);

but not this;

 test = function(message) {
 console.log(message);
 }
 setTimeout(test('xxx'), 3000);

I know it should be written like this; But the version above would be so much more convenient...

 test = function(message) {
 console.log(message);
 }
 setTimeout(function() { test('xxx'); }, 3000);

Answer 1

You're assigning the returned result of test to the callback argument of setTimeout instead of passing the function reference.

function callback(callback) {
 callback();
}
function say(message) {
 console.log(message);
 // this function doesn't return anything
}
// callback(say('Hello World!')) === callback() because say(message) doesnt return anything.

Answer 2

In your 2nd example it is immediately calling test with the string xxx.

Answer 3

You are calling the function immediatly in your second example. To pass the string, you can bind the parameter instead of calling it on its own:

setTimeout(test.bind(null,'xxxx'), 3000);

 test = function(message) {
 alert(message);
 }
 setTimeout(test.bind(null,'xxxx'), 3000);

Answer 4

If you write a function name with parenthesis, it will call it instead giving reference.

Answer 5

The second one doesn't work because setTimeout requires a function as a parameter (which it will call) and the second version you show has test('xxx') which means "the result of calling test('xxx')" which is undefined - not a function.