== Why is this an issue?

The ``++in++`` operator used on an array is valid but the code will likely not have the expected behavior. The ``++in++`` operator deals with the indexes of the array, not with the values.


If checking for an array slot is indeed desired, using ``++hasOwnProperty++`` makes the code intention clearer.


=== Noncompliant code example

[source,javascript]
----
function func1() {
    let arr = ["a", "b", "c"];

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

function func2() {
    let arr = ["a", "b", "c"];

    let expectedValue = "1"; // index #1 is corresponding to the value "b"
    if (expectedValue in arr) { // Noncompliant, will be always true because the array is made of 3 elements and the #1 is always there whatever its value
        return expectedValue + " found in the array";
    } else {
        return expectedValue + " not found";
    }
}
----


=== Compliant solution

[source,javascript]
----
function func() {
    let arr = ["a", "b", "c"];

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



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[]