JavaScript: What is exactly Declaring, Defining and initialising a variable or a function

Question

I'm trying to understand what means exactly declaring, defining and initialising of a variable.

let a;      // If this is a declaration

let a = 22; // and this is initialising

how I define a variable?

can I initialise a function?

Answer 1

how I define a variable?

With a variable declaration. There is no distinction between defining a variable and declaring a variable. Well, okay, there's one case of defining but not declaring a variable: Assigning to an undeclared identifier in loose mode, which is what I call The Horror of Implicit Globals because it creates a global variable without any declaration. Don't do that. :-)

How I declare a function?

My answer here to another question lists the various ways you can create functions in JavaScript. One of those ways is a function declaration.

Some quick examples, but I doubt that question or that answer will get deleted:

Function Declaration:

function foo() {
}

Note we're not doing x = in front of it. Anything that would great that as an expression would make it not a declaration anymore.

"Anonymous" function Expression (which despite the term, sometimes create functions with names):

x = function() { };
//  ^^^^^^^^^^^^^^--- this is the expression

doSomethingWith(function() { });
//              ^^^^^^^^^^^^^^--- this is the expression

Named function Expression

x = function foo() { };
//  ^^^^^^^^^^^^^^^^^^--- this is the expression

doSomethingWith(function foo() { });
//              ^^^^^^^^^^^^^^^^^^--- this is the expression

Accessor Function Initializer (ES5+):

obj = {
    get foo() {         // << This is an accessor, specifically a getter
    },                  // <<
    set foo() {         // << This is an accessor, specifically a setter
    }                   // <<
}

Arrow Function Expression (ES2015+) (which, like anonymous function expressions, don't involve an explicit name, and yet can create functions with names); these can be verbose (with a { .. } body) or concise (without {}):

x = () => { };
//  ^^^^^^^^^--- Verbose arrow function

doSomethingWith(() => { });
//              ^^^^^^^^^--- another verbose arrow function

y = () => expressionGoesHere;
//  ^^^^^^^^^^^^^^^^^^^^^^^^--- Concise arrow function

doSomethingWith(() => expressionGoesHere);
//              ^^^^^^^^^^^^^^^^^^^^^^^^--- another concise arrow function

Method Declaration in Object Initializer (ES2015+)

obj = {
    foo() { // << This is the method declaration
    }       // <<
};

Constructor and Method Declarations in class (ES2015+)

class Example {
    foo() { // << This is the method declaration
    }       // <<
}

can I initialise a function?

You can initialize a variable (or property) with a reference to a function:

let a = function() { };           // Variable
someObject.b = function() { };    // Object property