The JavaScript array.join() method

Q: How do I turn an array into a string in JavaScript?

Use arr.join(separator), e.g. arr.join(", "). With no argument it joins with commas.

Q: What is the difference between join() and split()?

join() combines an array into a string; split() breaks a string into an array. They're opposites.

Q: How do I join an array with no separator?

Pass an empty string: arr.join("").

Method JavaScript All modern browsers Updated June 23, 2026

Quick answer

The join() method turns an array into a string, placing a separator of your choice between elements. ["a", "b", "c"].join("-") gives "a-b-c". With no argument it uses a comma. It's the natural counterpart to split(), which goes the other way, from string back to array.

Overview

join() flattens an array into a single string. You give it the glue — the separator to put between elements — and it stitches everything together. join(", ") for a readable list, join("") to mash elements together with nothing between, join(" / ") for a breadcrumb. Pass no argument and it defaults to a comma.

It's the mirror image of split(): split breaks a string into an array, join glues an array into a string. The two together are a common idiom for transforming text — split on spaces, map over the words, join back up. The original array isn't changed; you just get a string out.

One quirk worth knowing: null and undefined elements become empty strings in the output, so [1, null, 3].join("-") gives "1--3". Everything else is converted with its normal string form. If you just want the default comma-separated version, toString() does the same thing, but join() is clearer because it shows the separator.

Syntax

const str = array.join()          // comma-separated by default
const str = array.join(separator) // your separator between elements

Parameters

The array.join() method accepts the following parameters.

Parameter	Description
`separator`	Optional. The string placed between elements. Defaults to a comma (`","`). Use `""` for no separator.

Example

Live example

<pre id="out" style="font:15px ui-monospace,monospace"></pre>
<script>
  const tags = ['html', 'css', 'js'];

  document.getElementById('out').textContent =
    tags.join(', ') + '\n' +
    tags.join(' > ') + '\n' +
    tags.join('');
  // html, css, js / html > css > js / htmlcssjs
</script>

Best practices

Pass an explicit separator — join(", ") or join("") — so the output is obvious from the code.
Pair it with split() to round-trip between strings and arrays.
Remember null and undefined become empty strings, which can leave doubled separators.
Build HTML or CSV lists by mapping first, then joining — items.map(...).join("").

Frequently asked questions

How do I turn an array into a string in JavaScript?

Use arr.join(separator), e.g. arr.join(", "). With no argument it joins with commas.

What is the difference between join() and split()?

join() combines an array into a string; split() breaks a string into an array. They're opposites.

How do I join an array with no separator?

Pass an empty string: arr.join("").

What happens to null or undefined when joining?

They're turned into empty strings, so you can end up with consecutive separators, e.g. [1, null, 2].join("-") is "1--2".

Array.isArray() array.map()

References