function f(a, b, ...theArgs) {  // …}

There are some additional syntax restrictions:

• A function definition can only have one rest parameter.
• The rest parameter must be the last parameter in the function definition.
• Trailing commas are not allowed after the rest parameter.
• The rest parameter cannot have adefault value.

A function definition's last parameter can be prefixed with... (three U+002E FULL STOP characters), which will cause all remaining (user supplied) parameters to be placed within anArray object.

function myFun(a, b, ...manyMoreArgs) {  console.log("a", a);  console.log("b", b);  console.log("manyMoreArgs", manyMoreArgs);}myFun("one", "two", "three", "four", "five", "six");// Console Output:// a, one// b, two// manyMoreArgs, ["three", "four", "five", "six"]

The rest parameter may bedestructured, which allows you to ignore certain parameter positions.

function ignoreFirst(...[, b, c]) {  return b + c;}

However, the following are all syntax errors:

function wrong1(...one, ...wrong) {}function wrong2(...wrong, arg2, arg3) {}function wrong3(...wrong,) {}function wrong4(...wrong = []) {}

The rest parameter is not counted towards the function'slength property.

There are four main differences between rest parameters and thearguments object:

• Thearguments object isnot a real array, while rest parameters areArray instances, meaning methods likesort(),map(),forEach() orpop() can be applied on it directly.
• Thearguments object has the additional (deprecated)callee property.
• In a non-strict function with simple parameters, thearguments objectsyncs its indices with the values of parameters. The rest parameter array never updates its value when the named parameters are re-assigned.
• The rest parameter bundles all theextra parameters into a single array, but does not contain any named argument definedbefore the...restParam. Thearguments object contains all of the parameters — including the parameters in the...restParam array — bundled into one array-like object.

In this example, the first argument is mapped toa and the second tob, so these named arguments are used as normal.

However, the third argument,manyMoreArgs, will be an array that contains the third, fourth, fifth, sixth, …, nth — as many arguments as the user specifies.

function myFun(a, b, ...manyMoreArgs) {  console.log("a", a);  console.log("b", b);  console.log("manyMoreArgs", manyMoreArgs);}myFun("one", "two", "three", "four", "five", "six");// a, "one"// b, "two"// manyMoreArgs, ["three", "four", "five", "six"] <-- an array

Below, even though there is just one value, the last argument still gets put into an array.

// Using the same function definition from example abovemyFun("one", "two", "three");// a, "one"// b, "two"// manyMoreArgs, ["three"] <-- an array with just one value

Below, the third argument isn't provided, butmanyMoreArgs is still an array (albeit an empty one).

// Using the same function definition from example abovemyFun("one", "two");// a, "one"// b, "two"// manyMoreArgs, [] <-- still an array

Below, only one argument is provided, sob gets the default valueundefined, butmanyMoreArgs is still an empty array.

// Using the same function definition from example abovemyFun("one");// a, "one"// b, undefined// manyMoreArgs, [] <-- still an array

SincetheArgs is an array, a count of its elements is given by thelength property. If the function's only parameter is a rest parameter,restParams.length will be equal toarguments.length.

function fun1(...theArgs) {  console.log(theArgs.length);}fun1(); // 0fun1(5); // 1fun1(5, 6, 7); // 3

In the next example, a rest parameter is used to collect all parameters after the first parameter into an array. Each one of the parameter values collected into the array is then multiplied by the first parameter, and the array is returned:

function multiply(multiplier, ...theArgs) {  return theArgs.map((element) => multiplier * element);}const arr = multiply(2, 15, 25, 42);console.log(arr); // [30, 50, 84]

Array methods can be used on rest parameters, but not on thearguments object:

function sortRestArgs(...theArgs) {  const sortedArgs = theArgs.sort();  return sortedArgs;}console.log(sortRestArgs(5, 3, 7, 1)); // 1, 3, 5, 7function sortArguments() {  const sortedArgs = arguments.sort();  return sortedArgs; // this will never happen}console.log(sortArguments(5, 3, 7, 1));// throws a TypeError (arguments.sort is not a function)

Rest parameters were introduced to reduce the boilerplate code that was commonly used for converting a set of arguments to an array.

Before rest parameters,arguments need to be converted to a normal array before calling array methods on them:

function fn(a, b) {  const normalArray = Array.prototype.slice.call(arguments);  // — or —  const normalArray2 = [].slice.call(arguments);  // — or —  const normalArrayFrom = Array.from(arguments);  const first = normalArray.shift(); // OK, gives the first argument  const firstBad = arguments.shift(); // ERROR (arguments is not a normal array)}

Now, you can easily gain access to a normal array using a rest parameter:

function fn(...args) {  const normalArray = args;  const first = normalArray.shift(); // OK, gives the first argument}

• Functions guide
• Functions
• Spread syntax (...)
• Default parameters
• arguments
• Array