1st June 2020

Rest parameters and spread syntax

Many JavaScript built-in functions support an arbitrary number of arguments.

For instance:

  • Math.max(arg1, arg2, ..., argN) – returns the greatest of the arguments.
  • Object.assign(dest, src1, ..., srcN) – copies properties from src1..N into dest.
  • …and so on.

In this chapter we’ll learn how to do the same. And also, how to pass arrays to such functions as parameters.

Rest parameters ...

A function can be called with any number of arguments, no matter how it is defined.

Like here:

function sum(a, b) {
  return a + b;
}

alert( sum(1, 2, 3, 4, 5) );

There will be no error because of “excessive” arguments. But of course in the result only the first two will be counted.

The rest of the parameters can be included in the function definition by using three dots ... followed by the name of the array that will contain them. The dots literally mean “gather the remaining parameters into an array”.

For instance, to gather all arguments into array args:

function sumAll(...args) { // args is the name for the array
  let sum = 0;

  for (let arg of args) sum += arg;

  return sum;
}

alert( sumAll(1) ); // 1
alert( sumAll(1, 2) ); // 3
alert( sumAll(1, 2, 3) ); // 6

We can choose to get the first parameters as variables, and gather only the rest.

Here the first two arguments go into variables and the rest go into titles array:

function showName(firstName, lastName, ...titles) {
  alert( firstName + ' ' + lastName ); // Julius Caesar

  // the rest go into titles array
  // i.e. titles = ["Consul", "Imperator"]
  alert( titles[0] ); // Consul
  alert( titles[1] ); // Imperator
  alert( titles.length ); // 2
}

showName("Julius", "Caesar", "Consul", "Imperator");
The rest parameters must be at the end

The rest parameters gather all remaining arguments, so the following does not make sense and causes an error:

function f(arg1, ...rest, arg2) { // arg2 after ...rest ?!
  // error
}

The ...rest must always be last.

The “arguments” variable

There is also a special array-like object named arguments that contains all arguments by their index.

For instance:

function showName() {
  alert( arguments.length );
  alert( arguments[0] );
  alert( arguments[1] );

  // it's iterable
  // for(let arg of arguments) alert(arg);
}

// shows: 2, Julius, Caesar
showName("Julius", "Caesar");

// shows: 1, Ilya, undefined (no second argument)
showName("Ilya");

In old times, rest parameters did not exist in the language, and using arguments was the only way to get all arguments of the function. And it still works, we can find it in the old code.

But the downside is that although arguments is both array-like and iterable, it’s not an array. It does not support array methods, so we can’t call arguments.map(...) for example.

Also, it always contains all arguments. We can’t capture them partially, like we did with rest parameters.

So when we need these features, then rest parameters are preferred.

Arrow functions do not have "arguments"

If we access the arguments object from an arrow function, it takes them from the outer “normal” function.

Here’s an example:

function f() {
  let showArg = () => alert(arguments[0]);
  showArg();
}

f(1); // 1

As we remember, arrow functions don’t have their own this. Now we know they don’t have the special arguments object either.

Spread syntax

We’ve just seen how to get an array from the list of parameters.

But sometimes we need to do exactly the reverse.

For instance, there’s a built-in function Math.max that returns the greatest number from a list:

alert( Math.max(3, 5, 1) ); // 5

Now let’s say we have an array [3, 5, 1]. How do we call Math.max with it?

Passing it “as is” won’t work, because Math.max expects a list of numeric arguments, not a single array:

let arr = [3, 5, 1];

alert( Math.max(arr) ); // NaN

And surely we can’t manually list items in the code Math.max(arr[0], arr[1], arr[2]), because we may be unsure how many there are. As our script executes, there could be a lot, or there could be none. And that would get ugly.

Spread syntax to the rescue! It looks similar to rest parameters, also using ..., but does quite the opposite.

When ...arr is used in the function call, it “expands” an iterable object arr into the list of arguments.

For Math.max:

let arr = [3, 5, 1];

alert( Math.max(...arr) ); // 5 (spread turns array into a list of arguments)

We also can pass multiple iterables this way:

let arr1 = [1, -2, 3, 4];
let arr2 = [8, 3, -8, 1];

alert( Math.max(...arr1, ...arr2) ); // 8

We can even combine the spread syntax with normal values:

let arr1 = [1, -2, 3, 4];
let arr2 = [8, 3, -8, 1];

alert( Math.max(1, ...arr1, 2, ...arr2, 25) ); // 25

Also, the spread syntax can be used to merge arrays:

let arr = [3, 5, 1];
let arr2 = [8, 9, 15];

let merged = [0, ...arr, 2, ...arr2];

alert(merged); // 0,3,5,1,2,8,9,15 (0, then arr, then 2, then arr2)

In the examples above we used an array to demonstrate the spread syntax, but any iterable will do.

For instance, here we use the spread syntax to turn the string into array of characters:

let str = "Hello";

alert( [...str] ); // H,e,l,l,o

The spread syntax internally uses iterators to gather elements, the same way as for..of does.

So, for a string, for..of returns characters and ...str becomes "H","e","l","l","o". The list of characters is passed to array initializer [...str].

For this particular task we could also use Array.from, because it converts an iterable (like a string) into an array:

let str = "Hello";

// Array.from converts an iterable into an array
alert( Array.from(str) ); // H,e,l,l,o

The result is the same as [...str].

But there’s a subtle difference between Array.from(obj) and [...obj]:

  • Array.from operates on both array-likes and iterables.
  • The spread syntax works only with iterables.

So, for the task of turning something into an array, Array.from tends to be more universal.

Get a new copy of an array/object

Remember when we talked about Object.assign() in the past?

It is possible to do the same thing with the spread syntax.

let arr = [1, 2, 3];
let arrCopy = [...arr]; // spread the array into a list of parameters
                        // then put the result into a new array

// do the arrays have the same contents?
alert(JSON.stringify(arr) === JSON.stringify(arrCopy)); // true

// are the arrays equal?
alert(arr === arrCopy); // false (not same reference)

// modifying our initial array does not modify the copy:
arr.push(4);
alert(arr); // 1, 2, 3, 4
alert(arrCopy); // 1, 2, 3

Note that it is possible to do the same thing to make a copy of an object:

let obj = { a: 1, b: 2, c: 3 };
let objCopy = { ...obj }; // spread the object into a list of parameters
                          // then return the result in a new object

// do the objects have the same contents?
alert(JSON.stringify(obj) === JSON.stringify(objCopy)); // true

// are the objects equal?
alert(obj === objCopy); // false (not same reference)

// modifying our initial object does not modify the copy:
obj.d = 4;
alert(JSON.stringify(obj)); // {"a":1,"b":2,"c":3,"d":4}
alert(JSON.stringify(objCopy)); // {"a":1,"b":2,"c":3}

This way of copying an object is much shorter than let objCopy = Object.assign({}, obj); or for an array let arrCopy = Object.assign([], arr); so we prefer to use it whenever we can.

Summary

When we see "..." in the code, it is either rest parameters or the spread syntax.

There’s an easy way to distinguish between them:

  • When ... is at the end of function parameters, it’s “rest parameters” and gathers the rest of the list of arguments into an array.
  • When ... occurs in a function call or alike, it’s called a “spread syntax” and expands an array into a list.

Use patterns:

  • Rest parameters are used to create functions that accept any number of arguments.
  • The spread syntax is used to pass an array to functions that normally require a list of many arguments.

Together they help to travel between a list and an array of parameters with ease.

All arguments of a function call are also available in “old-style” arguments: array-like iterable object.

Tutorial map

Comments

read this before commenting…
  • If you have suggestions what to improve - please submit a GitHub issue or a pull request instead of commenting.
  • If you can't understand something in the article – please elaborate.
  • To insert few words of code, use the <code> tag, for several lines – wrap them in <pre> tag, for more than 10 lines – use a sandbox (plnkr, jsbin, codepen…)