Why do functions return `undefined` instead of `null` by default?

Question

Why do JavaScript functions return undefined by default instead of null? Is this a totally arbitrary choice by the specification, or is there a larger ECMAScript-behavior context in which this particular choice can be understood?

function a() {}
a();
// undefined

What is the difference between null and undefined? Is there a specification-based reason why is undefined more appropriate as a default return value, or was it an arbitrary choice?

Answer 1

The specification says of null and undefined:

undefined value

primitive value used when a variable has not been assigned a value

null value

primitive value that represents the intentional absence of any object value

undefined represents a failure to assign a value. It is the total absence of a value. null represents the positive assertion of a non-value in an object context. null is intended to be used when an object is expected but the current value is no-object.

Given these two definitions, it seems obvious that undefined is the correct choice, since

1. functions can return values other than objects, and
2. a failure to specify a return value maps neatly onto a failure to assign a value

Answer 2

That's part of the specification. If no explicit return value is returned from a given function, the return value will always be undefined.

When a return statement is called in a function, the execution of this function is stopped. If specified, a given value is returned to the function caller. If the expression is omitted, undefined is returned instead

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/return

Answer 3

Few key statements to answer your question

JavaScript does not have a void type, so every function must return a value. The default value is undefined, except for constructors, where the default return value is this.

undefined and null are two distinct types: undefined is a type itself (undefined) while null is defined.

So, if you have returned nothing then it has to be nothing i.e. undefined.

Answer 4

The ECMAScript specification explicitly states that functions return undefined if no other return is specified. It's the default behaviour of the language.

See the last step in the [[Call]] internal method specification:

9.2.1 [[Call]] ( thisArgument, argumentsList)

The [[Call]] internal method for an ECMAScript function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

…

11. Return NormalCompletion(undefined).

Note that null and undefined are two distinct values in JavaScript. Again, according the the specification:

4.3.10 undefined value

primitive value used when a variable has not been assigned a value

4.3.12 null value

primitive value that represents the intentional absence of any object value