As for interacting with the DOM, different browsers will offer methods -and results- that are different between them to some or other extent.
But how about JavaScript itself? Do all browsers (or platforms e.g. Adobe Reader) implement the same syntax and behavior?
Aspects that particularly worry me are function/object handling and arithmetical operations.
Best Answer
The JavaScript standards committee updates JavaScript annually. Browser Support Varies. Tooling is available to help.
Javascript is defined by the ECMA-262 standard, which is updated anually. Every revision has added some new syntax. All browsers since IE5 support a common subset of syntax in the same way, it's only a question of if they support new syntax options or not. However, since syntax extensions can be as small as Allowing trailing commas in parameter lists, it's not always obvious which options you are using. Here's a list of the changes since they went to annual updates.
It is common practice (in 2021,) to include a build step for your JavaScript that replaces newer syntax with older syntax so that code can be run in older browsers that are not updated. Babel, Browserslist, and @babel/preset-env make it easy to support a configurable set of browsers. (As long as you're still developing your site!)