What are first class functions?

In Javascript, functions are first-class objects. First-class functions mean when functions in that language are treated like any other variable.

For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function, and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener

const handler = () => console.log ('This is a click handler function');

document.addEventListener ('click', handler);