language_optional_labeled_argument.mdx

id	keywords	name	summary	category
optional-labeled-argument	optional labeled argument	~arg=?	This is an `optional labeled argument`.	languageconstructs

Labeled arguments, i.e. arguments that are prefixed with ~, can be suffixed with =? to denote that they are optional. Thus, they can be omitted when calling the function.

Example

<CodeTab labels={["ReScript", "JS Output"]}>

let print = (text, ~logLevel=?) => {
  switch logLevel {
  | Some(#error) => Console.error(text)
  | _ => Console.log(text)
  }
}

print("An info")
print("An error", ~logLevel=#error)

function print(text, logLevel) {
  if (logLevel === "error") {
    console.error(text);
  } else {
    console.log(text);
  }
}

print("An info", undefined);

print("An error", "error");

References

• Labeled Arguments
• Optional Labeled Arguments
• Function Syntax Cheatsheet