== Why is this an issue?

Functions that accept random numbers of arguments use a rest argument (``++... argname++``). This allows you to pass the function each relevant parameter individually. At runtime, those parameters are automatically wrapped in an array. If you pass an array to such a function, it will automatically be re-wrapped in another array, and this double-layering won't be expected by the called function. To avoid that double-layering, use the spread operator (``++...++``arrayToBeExpanded) in the call to expand the array.


This applies both to manually created arrays, and to arguments that were accepted as rest parameters.


=== Noncompliant code example

[source,javascript]
----
collect(new Book(), new Book());

function collect(...books: Book[]) {
  buy(books);   // Noncompliant
}

function buy(...things: any[]) {
  console.log(things); // outputs "[ [ Book {}, Book {} ] ]"
}
----


=== Compliant solution

[source,javascript]
----
collect(new Book(), new Book());

function collect(...books: Book[]) {
  buy(...books);
}

function buy(...things: any[]) {
  console.log(things); // outputs "[ Book {}, Book {} ]"
}
----



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

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

=== Message

Use spread operator '...' to pass this argument


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

=== on 3 Jan 2018, 20:37:24 Ann Campbell wrote:
\[~elena.vilchik] a little text on what "spread" is and why it's necessary (when the argument you're passing is apparently already an array) would be nice.

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