JS: “function” (keyword) declaration vs expression

When is function definition a declaration?

It is a function declaration if all of the following are met:

• At top-level of source code (that is, not inside any curly bracket.) or it is at top-level inside a function.
• Not on the right-hand-side of a assignment.

/* Function Declaration */
function ff() {
 return 3;
}
console.log(ff());
// 3

Function declaration must be top level

Named function form is a declaration only if it is at top level of source code or top level inside a function body. When in elsewhere, its a named function expression.

// no function declaration happens here. The ff is treated as named function expression
const gg = function ff() {
 return 3;
};

console.log(ff());

// error: Uncaught (in promise) ReferenceError: ff is not defined

When is function expression

When you use the keyword function, and it is not at top-level of source code, nor at top-level inside a function, it has meaning of Function Expression.

// function expression, applied on the spot
console.log(
 (function (x) {
  return x;
 })(4),
);
// 4

🟢 tip: i recommend don't use the keyword function. use Arrow Function.