The ability to define default values for function parameters can make a function easier to use. Default parameter values allow callers to specify as many or as few arguments as they want while getting the same functionality and minimizing boilerplate, wrapper code.


But all function parameters with default values should be declared after the function parameters without default values. Otherwise, it makes it impossible for callers to take advantage of defaults; they must re-specify the defaulted values or pass ``++undefined++`` in order to "get to" the non-default parameters.

== Noncompliant Code Example

[source,javascript]
----
function multiply(a = 1, b) {  // Noncompliant
  return a*b;
}

var x = multiply(42);  // returns NaN as b is undefined
----

== Compliant Solution

[source,javascript]
----
function multiply(b, a = 1) {
  return a*b;
}

var x = multiply(42);  // returns 42 as expected
----


== Exceptions

When writing Redux reducers, there is a convention to use default argument syntax to provide initial state (first argument), while action (second argument) is mandatory. A reducer may be called with ``++undefined++`` as the state value when the application is being initialized.

[source,javascript]
----
// Use the initialState as a default value
export default function appReducer(state = initialState, action) {
  switch (action.type) {
    default:
      return state;
  }
}
----
=== See

* https://redux.js.org/tutorials/fundamentals/part-3-state-actions-reducers#writing-reducers[Writing Reducers] - Redux documentation

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== Implementation Specification
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include::message.adoc[]

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