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join() in JS – How to Join Array into String

Whenever you use join() on an array, the method does the following:

  1. It uses commas (or a user-defined value) to separate the array items.
  2. It joins all the array elements together into a new string.
  3. It returns the newly created string—without changing the original array.
note

join() is sometimes written as Array.prototype.join() because it is a method of the Array object's prototype property.

Syntax of the join() Method​

join() accepts only one argument. Here is the syntax:

arrayObject.join(separator);

Note the following:

  • A comma (,) is join()'s default separator. Therefore, if you do not specify a separator argument, browsers will use commas to separate the array's elements.
  • Suppose the array has only one element. In that case, browsers will ignore the separator argument and return only the array item.
  • If the separator is an empty string (""), JavaScript will join the array's elements without using any character to separate them.

Examples​

Below are examples of the join() method.

How to join array elements with commas​

// Define an array:
const bestColors = ["White", "Pink", "Blue"];

// Join the bestColors array together with a comma separator:
const joinedString = bestColors.join();

// Log joinedString to the console:
console.log(joinedString);

// The invocation above will return: "White,Pink,Blue"

Try it on StackBlitz

In the example above, the browser automatically used a comma (,) as the separator because we did not pass any argument into the join() method.

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How to join array elements with ", and "​

// Define an array:
const bestColors = ["White", "Pink", "Blue"];

// Join the bestColors array together with a ", and " separator:
const joinedString = bestColors.join(", and ");

// Log joinedString to the console:
console.log(joinedString);

// The invocation above will return: "White, and Pink, and Blue"

Try it on StackBlitz

How to join array elements without any separator​

// Define an array:
const bestColors = ["White", "Pink", "Blue"];

// Join the bestColors array together without separating them with a character:
const joinedString = bestColors.join("");

// Log joinedString to the console:
console.log(joinedString);

// The invocation above will return: "WhitePinkBlue"

Try it on StackBlitz

How to join array elements with " *** "​

// Define an array:
const bestColors = ["White", "Pink", "Blue"];

// Join the bestColors array together with a " *** " separator:
const joinedString = bestColors.join(" *** ");

// Log joinedString to the console:
console.log(joinedString);

// The invocation above will return: "White *** Pink *** Blue"

Try it on StackBlitz

Important Stuff to Know about the join() Method​

Here are two essential facts to remember when using the join() method.

1. join() converts undefined and null to empty strings​

Suppose your array object contains an undefined or null element. In that case, join() will convert such values to empty strings instead of "undefined" or "null".

Here's an example:

// Define an array:
const bestColors = ["White", undefined, "Pink", null, "Blue"];

// Join bestColors array together with a "123" separator:
const joinedString = bestColors.join("123");

// Log joinedString to the console:
console.log(joinedString);

// The invocation above will return: "White123123Pink123123Blue"

Try it on StackBlitz

2. join() parses empty slots as undefined​

Suppose your array object has an empty slot. In that case, join() will treat the slot as if it contains an undefined value.

Here's an example:

// Define an array:
const bestColors = ["White", , "Pink", "Blue"];

// Join the bestColors array together with a "123" separator:
const joinedString = bestColors.join("123");

// Log joinedString to the console:
console.log(joinedString);

// The invocation above will return: "White123123Pink123Blue"

Try it on StackBlitz

Overview​

join() creates a new string by using a comma, or a user-defined separator, to join the elements of an array object.