<!DOCTYPE qhelp PUBLIC

"-//Semmle//qhelp//EN"

"qhelp.dtd">

<qhelp>

<overview>

<p>

Unlike Java and other statically typed languages, JavaScript does not require that all execution paths in

a function return a value: if an invocation of the function "falls off" the end of the function without

explicitly returning a value, the special value <code>undefined</code> is implicitly returned.

</p>

<p>

Relying on this behavior is dangerous, however, since it can lead to situations where <code>undefined</code>

is returned by accident, simply because the programmer overlooked a possible execution path. Functions

with implicit return behavior may also be hard to use in client code, which has to be prepared for

either an explicit return value or <code>undefined</code> being returned.

</p>

</overview>

<recommendation>

<p>

Insert an explicit return statement. If the "fall through" behavior should never be triggered, it is often

a good idea to instead throw an exception.

</p>

</recommendation>

<example>

<p>

In the following example, the function <code>call</code> takes two arguments <code>o</code> and <code>m</code>.

It checks whether <code>o</code> has a function-valued property with name <code>m</code>, and if so invokes

it as a method, returning the result. Otherwise, nothing is returned.

</p>

<sample src="examples/ImplicitReturn.js" />

<p>

Assuming that the second case should never occur, an exception should be thrown as follows:

</p>

<sample src="examples/ImplicitReturnGood.js" />

</example>

<references>

<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/return">Return</a>.</li>

</references>

</qhelp>