Unlike in JavaScript, where every parameter can be omitted, in TypeScript you need to explicitly declare this in the function signature. Either you add ``++?++`` in the parameter declaration and ``++undefined++`` will be automatically applied to this parameter. Or you add an initializer with a default value in the parameter declaration. In the latter case, when passing ``++undefined++`` for such parameter, default value will be applied as well. So it's better to avoid passing ``++undefined++`` value to an optional or default parameter because it creates more confusion than it brings clarity. Note, that this rule is only applied to the last arguments in function call.


== Noncompliant Code Example

----
function foo(x: number, y: string = "default", z?: number) {
  // ...
}

foo(42, undefined); // Noncompliant
foo(42, undefined, undefined); // Noncompliant
foo(42, undefined, 5); // OK, there is no other way to force default value for second parameter
----


== Compliant Solution

----
function foo(x: number, y: string = "default", z?: number) {
  // ...
}

foo(42);
----


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== Implementation Specification
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