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

Alex Francis
Alex Francis
11,625 Points

Variable 'headline' not defined, but function still runs! Mysterious..

Hello,

With this code:

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

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

... the headline does indeed get a red border when the 'Update Heading' button is pressed.

However, the variable 'headline' in the Javascript has not been defined!

Typing 'headline' in the console returns the correct h1 element: <h1 id="headline" style="border: 2px solid red;">My Day</h1>

I can't figure this one out! Does anyone else know?

Is it something to do with browser cache keeping the 'headline' variable maybe?

1 Answer

Jason Larson
Jason Larson
7,734 Points

This works because you are modifying an element that has an id of "headline". It wouldn't work on a class or element, or if the name of the id doesn't conform to standard variable naming (like "btn-main"). Ids are created by default as properties of the global object (in global context). (Please don't ask me where this is documented, as I don't know. I found it googling around for the answer to your question.) Basically, ids act very similar to a global variable. You should avoid this use of them, though, as you could run into issues that are difficult to troubleshoot (like if you had a global variable named "headline", as it would shadow the "headline" property).

Alex Francis
Alex Francis
11,625 Points

Woah, that's crazy! So in effect when the browser renders the HTML into a DOM, it assigns elements with an ID to variables..

I suppose this has something to do with the fact you can navigate a webpage using the ID values in the url..

I just checked and yeah you can reassign the variable 'headline'; I made it into a string ('hello!'). You can even redefine it as a const.

I wonder if the browser is creating this ID variable as a var? Rather than let or const.

Thanks for your answer, that was very instructive :)

I don't imagine there is much one can do specifically with these ID variables (properties of the global object, like you say), but it's interesting to know that they exist.