Issue
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?
Solution
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
- functions can return values other than objects, and
- a failure to specify a return value maps neatly onto a failure to assign a value
Answered By – apsillers
Answer Checked By – Robin (BugsFixing Admin)