Functions - JavaScript | MDN

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Generally speaking, a function is a "subprogram" that can be called by code external (or internal, in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function as parameters, and the function will return a value.
In JavaScript, functions are first-class objects, because they can be passed to other functions, returned from functions, and assigned to variables and properties. They can also have properties and methods just like any other object. What distinguishes them from other objects is that functions can be called.
For more examples and explanations, see the JavaScript guide about functions.
Function values are typically instances of Function. See Function for information on properties and methods of Function objects. Callable values cause typeof to return "function" instead of "object".
Note: Not all callable values are instanceof Function. For example, the Function.prototype object is callable but not an instance of Function. You can also manually set the prototype chain of your function so it no longer inherits from Function.prototype. However, such cases are extremely rare.
By default, if a function's execution doesn't end at a return statement, or if the return keyword doesn't have an expression after it, then the return value is undefined. The return statement allows you to return an arbitrary value from the function. One function call can only return one value, but you can simulate the effect of returning multiple values by returning an object or array and destructuring the result.
Note: Constructors called with new have a different set of logic to determine their return values.
Parameters and arguments have slightly different meanings, but in MDN web docs, we often use them interchangeably. For a quick reference:
In this example, the num variable is called the function's parameter: it's declared in the parenthesis-enclosed list of the function's definition. The function expects the num parameter to be a number — although this is not enforceable in JavaScript without writing runtime validation code. In the formatNumber(2) call, the number 2 is the function's argument: it's the value that is actually passed to the function in the function call. The argument value can be accessed inside the function body through the corresponding parameter name or the arguments object.
Arguments are always passed by value and never passed by reference. This means that if a function reassigns a parameter, the value won't change outside the function. More precisely, object arguments are passed by sharing, which means if the object's properties are mutated, the change will impact the outside of the function. For example:
The this keyword refers to the object that the function is accessed on — it does not refer to the currently executing function, so you must refer to the function value by name, even within the function body.
Broadly speaking, JavaScript has four kinds of functions:
For every kind of function, there are multiple ways to define it:
function, function*, async function, async function*
function, function*, async function, async function*
Function(), GeneratorFunction(), AsyncFunction(), AsyncGeneratorFunction()
In addition, there are special syntaxes for defining arrow functions and methods, which provide more precise semantics for their usage. Classes are conceptually not functions (because they throw an error when called without new), but they also inherit from Function.prototype and have typeof MyClass === "function".
All syntaxes do approximately the same thing, but there are some subtle behavior differences.
For function expressions, there is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be different from the variable the function is assigned to — they have no relation to each other. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or gets another value, if the same name is declared elsewhere). For example:
On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared.
A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body.
A function defined by new Function will dynamically have its source assembled, which is observable when you serialize it. For example, console.log(new Function().toString()) gives:
This is the actual source used to compile the function. However, although the Function() constructor will create the function with name anonymous, this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error:
A function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a Function constructor does not inherit any scope other than the global scope (which all functions inherit).
Functions defined by function expressions and function declarations are parsed only once, while a function defined by the Function constructor parses the string passed to it eac