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JavaScript JavaScript and the DOM Getting a Handle on the DOM Select an Element by ID

Turandeep Munday
Turandeep Munday
6,047 Points
Posted by Turandeep Munday
Turandeep Munday
6,047 Points

When i move the function out of the event listener itself my page loads with the styles already applied

// we have a button object with id="btn-main" and <h1 id ="headline">

const headline = document.getElementById('headline');
const selectButtonElement = document.getElementById('btn-main');

function changeBorder(selection, borderString) {
  let a = selection;
  let b = borderString;
  a.style.border = b;  
}; 

selectButtonElement.addEventListener('click', changeBorder(headline, 'solid 2px red'));

My thinking is when i select the button element then the fucntion should run with selected params and it should run much in the same way as using an arrow function as second argument on .addEventListener

Turandeep Munday
Turandeep Munday
6,047 Points
Turandeep Munday
Turandeep Munday
6,047 Points

this is the code that works from the course 

const headline = document.getElementById('headline');
const selectButtonElement = document.getElementById('btn-main');

function changeBorder(selection, borderString) {
  let a = selection;
  let b = borderString;
  a.style.border = b;  
}; 

selectButtonElement.addEventListener('click', () => {
                                     headline.style.border = 'solid 2px red';
                                     })

1 Answer

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

When a function (like addEventListener) needs a callback argument, you should pass only a reference to the function. But what's happening in this code is that handler function is being called, and its return value (which is "undefined" in this case) is being passed as the callback. Calling the function is what applies the styles immediately.

In the original lesson code, the arrow function expression creates a reference and defines a function but the function isn't actually called until the event occurs.