If i'm writing event in this way, action executes when it should:

 document.getElementById('myElem').onmousedown = (e) => {
     console.log('fired!')
    }

But if i'm writing same stuff in other way, action executes when page is loaded, once:

let HandleEvent = (event) => {
 console.log('fired!')
}
document.getElementById('myElem').onmousedown = HandleEvent(event)

UPD: Ofcourse it is just example, HandleEvent function will have much more plex logic.

My questions is:

1. Why?
2. How to make it work properly?

If i'm writing event in this way, action executes when it should:

 document.getElementById('myElem').onmousedown = (e) => {
     console.log('fired!')
    }

But if i'm writing same stuff in other way, action executes when page is loaded, once:

let HandleEvent = (event) => {
 console.log('fired!')
}
document.getElementById('myElem').onmousedown = HandleEvent(event)

UPD: Ofcourse it is just example, HandleEvent function will have much more plex logic.

My questions is:

1. Why?
2. How to make it work properly?

• javascript

• 5 onmousedown expects a function reference, but what you're passing is the returned value of the function (which is undefined) because you're invoking it. Try document.getElementById('myElem').onmousedown = HandleEvent instead. – haim770 Commented Jul 7, 2016 at 11:12

6 Answers 6

document.getElementById('myElem').onmousedown = HandleEvent;

Would do what you want; you should not execute the handler when assigning it. You should just assign the function reference.

Then you can also assign it as the onload handler, or even call it yourself (if you are not relying on the event object)

document.getElementById('myElem').onmousedown = HandleEvent;

To pass additional arguments you could use bind:

document.getElementById('myElem').onmousedown = HandleEvent.bind(null, argument1, argument2);

...or you could use a factory function:

document.getElementById('myElem').onmousedown = createHandler();

function createHandler() {
  var a = calculateA();
  var b = calculateA();
  return function handleEvent() {
     //use a and/or b
  };
}

There are other ways too. Precise code will depend on your use case.

You don't need the event.

let HandleEvent = (event) => 'fired!';
document.getElementById('myElem').onmousedown = HandleEvent

Remove the event

let HandleEvent = (event) => {
 console.log('fired!')
}
document.getElementById('myElem').onmousedown = HandleEvent

If you want to pass an attribute

let HandleEvent = (event) => {
 console.log('fired!')
}
document.getElementById('myElem').onmousedown = HandleEvent.bind(attribute)

What you did was assigning the result of HandleEvent() to the event, instead you should assign the function to it.

Correct statement:

document.getElementById('myElem').onmousedown = HandleEvent;