rspec/rules/S2259/javascript/rule.adoc

== Why is this an issue?

In JavaScript, `null` and `undefined` are primitive values that do not have properties or methods. When accessing a property on a `null` or `undefined` value, JavaScript tries to access the property of an object that does not exist, which results in a `TypeError`.

This can cause the program to crash or behave unexpectedly, which can be difficult to debug.

== How to fix it

=== Code examples

==== Noncompliant code example

[source,javascript,diff-id=1,diff-type=noncompliant]
----
let foo = null;
console.log(foo.bar); // Noncompliant: TypeError will be thrown
----

==== Compliant solution

The simplest solution is to check in a `if` condition the equality to `null` or `undefined`.
With the https://262.ecma-international.org/6.0/#sec-abstract-equality-comparison[abstract equality operator] it is not
required to check both, as these operators consider `null` and `undefined` to be equals.

[source,javascript,diff-id=1,diff-type=compliant]
----
let foo;
if (foo != null) {
  console.log(foo.bar);
}
----

Also, the logical AND operator (`&&`) can be used to check if a variable is truthy before attempting to access its properties.
The expression will short-circuit and return the falsy value instead of attempting to access its properties.

[source,javascript,diff-id=1,diff-type=compliant]
----
let foo = null;
console.log(foo && foo.bar);
----

Alternatively, use the optional chaining operator (`?.`) to check if the variable is `null` or `undefined` before attempting to access its property. This operator is more readable and concise, especially when dealing with nested properties.

[source,javascript,diff-id=1,diff-type=compliant]
----
let foo = null;
console.log(foo?.bar);
----

== Resources

=== Documentation

* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined[`undefined`]
* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/null[`null`]
* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError[`TypeError`]
* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/No_properties[TypeError: "x" has no properties]
* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining[Optional chaining (``++?.++``)]
* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND[Logical AND (``++&&++``)]

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== Comments And Links
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=== on 15 Mar 2016, 09:45:14 Pierre-Yves Nicolas wrote:
Useful links:

* \https://javascriptweblog.wordpress.com/2010/08/16/understanding-undefined-and-preventing-referenceerrors/
* \https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined

We could make this rule also cover invalid function calls, e.g.:

----
var x = 42;
x(); // Noncompliant: TypeError will be thrown
----
However, there may be some overlap with RSPEC-2999.

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