Spread Operator: How Spread Works in JavaScript

The spread operator (...) is a syntax that helps to expand iterables into individual elements.

The spread syntax serves within array literals, function calls, and initialized properties object to spread the values of iterable objects into separate items.

Note

A spread syntax is effective only when used within array literals, function calls, or initialized properties objects.

So, what exactly does this mean? Lets see with some examples.

Spread Example 1: How spread works in an array literal

const myName = ["Sofela", "is", "my"];const aboutMe = ["Oluwatobi", ...myName, "name."];console.log(aboutMe);// The invocation above will return:[ "Oluwatobi", "Sofela", "is", "my", "name." ]

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The snippet above used spread (...) to copy myName array into aboutMe.

Note

• Alterations to myName will not reflect in aboutMe because the spread operator simply copied and paste myNames content into aboutMe without creating any reference back to the original array.

• Suppose we did not use the spread syntax to duplicate myNames content. For instance, if we had written const aboutMe = ["Oluwatobi", myName, "name."]. In such a case, the computer would have assigned a reference back to myName. As such, any change made in the original array would reflect in the duplicated one.

Spread Example 2: How to use spread to convert a string into individual array items

const myName = "Oluwatobi Sofela";console.log([...myName]);// The invocation above will return:[ "O", "l", "u", "w", "a", "t", "o", "b", "i", " ", "S", "o", "f", "e", "l", "a" ]

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In the snippet above, we used the spread syntax (...) within an array literal object ([...]) to expand myNames string value into individual items.

As such, "Oluwatobi Sofela" got expanded into [ "O", "l", "u", "w", "a", "t", "o", "b", "i", " ", "S", "o", "f", "e", "l", "a" ].

Spread Example 3: How the spread operator works in a function call

const numbers = [1, 3, 5, 7];function addNumbers(a, b, c, d) {  return a + b + c + d;}console.log(addNumbers(...numbers));// The invocation above will return:16

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In the snippet above, we used the spread syntax to spread the numbers arrays content across addNumbers()s parameters.

Suppose the numbers array had more than four items. In such a case, the computer will only use the first four items as addNumbers() argument and ignore the rest.

Heres an example:

const numbers = [1, 3, 5, 7, 10, 200, 90, 59];function addNumbers(a, b, c, d) {  return a + b + c + d;}console.log(addNumbers(...numbers));// The invocation above will return:16

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Heres another example:

const myName = "Oluwatobi Sofela";function spellName(a, b, c) {  return a + b + c;}console.log(spellName(...myName));      // returns: "Olu"console.log(spellName(...myName[3]));   // returns: "wundefinedundefined"console.log(spellName([...myName]));    // returns: "O,l,u,w,a,t,o,b,i, ,S,o,f,e,l,aundefinedundefined"console.log(spellName({...myName}));    // returns: "[object Object]undefinedundefined"

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Spread Example 4: How spread works in an object literal

const myNames = ["Oluwatobi", "Sofela"];const bio = { ...myNames, runs: "codesweetly.com" };console.log(bio);// The invocation above will return:{ 0: "Oluwatobi", 1: "Sofela", runs: "codesweetly.com" }

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In the snippet above, we used spread inside the bio object to expand myNames values into individual properties.

Important stuff to know about the spread operator

Keep these two essential pieces of info in mind whenever you choose to use the spread operator.

Info 1: Spread operators cant expand object literals values

Since a properties object is not an iterable object, you cannot use the spread syntax to expand its values.

However, you can use the spread operator to clone properties from one object into another.

Heres an example:

const myName = { firstName: "Oluwatobi", lastName: "Sofela" };const bio = { ...myName, website: "codesweetly.com" };console.log(bio);// The invocation above will return:{ firstName: "Oluwatobi", lastName: "Sofela", website: "codesweetly.com" };

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The snippet above used the spread operator to clone myNames content into the bio object.

Note

• The spread operator can expand iterable objects values only.

• An object is iterable only if it (or any object in its prototype chain) has a property with a @@iterator key.

• Array, TypedArray, String, Map, and Set are all built-in iterable types because they have the @@iterator property by default.

• A properties object is not an iterable data type because it does not have the @@iterator property by default.

• You can make a properties object iterable by adding @@iterator onto it.

Info 2: The spread operator does not clone identical properties

Suppose you used the spread operator to clone properties from object A into object B. And suppose object B contains properties identical to those in object A. In such a case, Bs versions will override those inside A.

Heres an example:

const myName = { firstName: "Tobi", lastName: "Sofela" };const bio = { ...myName, firstName: "Oluwatobi", website: "codesweetly.com" };console.log(bio);// The invocation above will return:{ firstName: "Oluwatobi", lastName: "Sofela", website: "codesweetly.com" };

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Observe that the spread operator did not copy myNames firstName property into the bio object because bio already contains a firstName property.

Wrapping it up

This article discussed what the spread operator is. We also looked at how spread works in array literals, function calls, and object literals.

Thanks for reading!