<!DOCTYPE qhelp PUBLIC

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

"qhelp.dtd">

<qhelp>

<overview>

<p>

If a class extends another class, its constructor needs to call the super constructor

before referencing <code>this</code> or accessing properties through <code>super</code>.

Failure to do so will cause a runtime error.

</p>

</overview>

<recommendation>

<p>

Insert a super constructor call.

</p>

</recommendation>

<example>

<p>

In the following example, class <code>A</code> extends class <code>B</code>, but

its constructor assigns to <code>this.x</code> without first invoking the super

constructor, which will cause a runtime error.

</p>

<sample language="javascript">

class A extends B {

constructor() { this.x = 42; }

}

</sample>

<p>

To prevent the error, a super constructor call should be inserted:

</p>

<sample language="javascript">

class A extends B {

constructor() { super(); this.x = 42; }

}

</sample>

</example>

<references>

<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes#Sub_classing_with_extends">Sub classing with extends</a>.</li>

</references>

</qhelp>