== Why is this an issue?

The ``++in++`` operator is used to check if a property is in an object or its https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain[prototype chain].

When used on an array, it will compare against the indexes of the array, not the values. This is likely not to be the expected behavior.

[source,javascript,diff-id=1,diff-type=noncompliant]
----
function func() {
    const arr = ["a", "b", "c"];

    const expectedValue = "b";
    if (expectedValue in arr) { // Noncompliant: will be always false
        return expectedValue + " found in the array";
    } else {
        return expectedValue + " not found";
    }
}
----

Use the method `Array.prototype.includes()` to determine whether an array contains a certain value. If the actual intention is to check for an array slot, use ``++Object.prototype.hasOwnProperty()++``.

[source,javascript,diff-id=1,diff-type=compliant]
----
function func() {
    const arr = ["a", "b", "c"];

    const expectedValue = "b";
    if (arr.includes(expectedValue)) {
        return expectedValue + " found in the array";
    } else {
        return expectedValue + " not found";
    }
}
----

== Resources
=== Documentation

* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in[`in` operator]
* MDN web docs - link:++https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in++[``++for...in++``]
* MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes[`Array.prototype.includes()`]


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Use "indexOf" or "includes" (available from ES2016) instead.


=== Highlighting

"in" expression


'''
== Comments And Links
(visible only on this page)

=== on 7 May 2018, 13:56:14 Stas Vilchik wrote:
\[~alexandre.gigleux] JavaScript is not a compiled language, so please remove the word "compile" from the description.

endif::env-github,rspecator-view[]
