JavaScript: Difference Between Function Declaration vs Function 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, it is called Function Declaration.
• Not on the right-hand-side of a assignment.

/* Function Declaration */

function f() {
  return 3;
}
console.log(f() === 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.

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

console.log(f() === 3);

/*
error: Uncaught ReferenceError: f is not defined
*/

/* [
error: Uncaught ReferenceError: Cannot access 'h' before initialization
] */
h();

const h = function h() {
  return 3;
};

/* [
error: Uncaught ReferenceError: bb is not defined
] */
bb();

const aa = function bb() {
  return 3;
};

/* [
error: Uncaught ReferenceError: bb is not defined
] */
const aa = function bb() {
  return 3;
};

bb();

When is Function Expression

When you have this:

function name (params) {body}

or

function (params) {body}

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 () {
    return 3;
  })() === 3,
);

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

You can assign a function to a variable. In effect, giving the function a name.

/* give a function expression a name */
const g = function () {
  return 3;
};

console.log(g() === 3);

Function Declaration Hoist Name and Value

• JavaScript: Function Name Hoisting

Tip: never use the keyword 「function」

• Xah JavaScript Style Guide for Functional Programers