What does join() do?

JavaScriptJunior~1 min read

The join() method in JavaScript is used to combine all array elements into one string, separating them with a specified separator.

Example:

javascript

const fruits = ["apple", "banana", "cherry"];
const result = fruits.join(", ");

console.log(result); // "apple, banana, cherry"

Each array element is turned into a string.
The separator ", " is inserted between elements.
If no separator is specified, a comma is used by default.

Syntax:

javascript

array.join(separator);

Parameter	Description
`separator`	(optional) a string to insert between elements. If not specified, `","` is used.

A few examples:

Without a separator:

javascript

[1, 2, 3].join("");
// "123"

With a space:

javascript

["JS", "is", "awesome"].join(" ");
// "JS is awesome"

With another character:

javascript

["2025", "10", "14"].join("-");
// "2025-10-14"

With empty elements:

javascript

[1, , 3].join("-");
// "1--3" (the empty slot is still counted)

Important:

The method does not change the original array.
All elements are implicitly converted to strings before being joined.
Returns a string, not an array.

Practical use example:

javascript

const tags = ["frontend", "javascript", "interview"];
const hashtagString = "#" + tags.join(" #");

console.log(hashtagString);
// "#frontend #javascript #interview"

Summary:

What it does	Converts an array into a string
Default separator	`","`
Returns	A string
Changes the array	No
Example	`["a", "b"].join("-") -> "a-b"`

In summary: join() is a way to assemble an array into a string, inserting the needed separator between elements.

Short Answer

Interview ready

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