Some libraries uses the given below syntax in Javascript. Is the syntax valid in Javascript? If not, how do the libraries handle it behind the scene.
const button = css`
position: fixed;
bottom: 10%;
left: 50%;
transform: translateX(-50%);
z-index: 200;
`;
Yes it is valid syntax in JS.
Example:
function fun(){
console.log('triggered..!');
}
fun``
prints "triggered..!" on console.
this is called "Tagged templates"
more information check here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#tagged_templates
when you add closed backticks at end of js variable it will treat it as function call, function will be called with arguments which you pass between backticks.