Why Do Functions Return `undefined` Instead Of `null` By Default?

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 i

Solution 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

Solution 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

Solution 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.

Solution 4:

null is a defined value, there are two states for any variable, defined and undefined.

var a; // declared but undefined, has nothing in it; does not mean its null
var b = null; // declared and defined as null;

Every function in javascript has a implicit return: return; like in any other language to mark the end of the function. So it returns with nothing, which if you try to log, it will give you undefined

Solution 5:

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