== Why is this an issue?

A common idiom in JavaScript to differentiate between two possible types is to check for the presence in the object of a member of the desired type. Usually, to simplify the code, a boolean function is created to check the type.

Typescript provides user defined type guard functions. These are just functions with a return type of ``++argumentName is SomeType++``. Such functions return ``++true++`` if the argument is of the specified type. One of the advantages of using such a function is that in a conditional block where the condition is a type guard, the compiler automatically performs the appropriate casts, so explicit casting becomes unnecessary.


This rule raises an issue when a boolean function checking for the type of its only argument can be replaced with a user-defined type guard function.


=== Noncompliant code example

[source,javascript]
----
function isSomething(x: BaseType) : boolean { // Noncompliant
  return (<Something>x).foo !== undefined;
}

if (isSomething(v)) {
  (<Something>v).foo();
}
----


=== Compliant solution

[source,javascript]
----
function isSomething(x: BaseType) : x is Something {
  return (<Something>x).foo !== undefined;
}

if (isSomething(v)) {
  v.foo();
}
----


== Resources

https://www.typescriptlang.org/docs/handbook/advanced-types.html[TypeScript advanced types]


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

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

=== Message

Change this boolean return type into a type predicate


=== Highlighting

The function declaration


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

=== on 14 Nov 2017, 21:23:12 Ann Campbell wrote:
\[~jeanchristophe.collet] from this description I don't understand what type guards do, and from the examples, I have no idea how they work.

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