How to Join Two or More Strings in JavaScript

Learn how to join two or more strings with JavaScript.

In JavaScript there are different methods to join two or more strings together, including the plus operator +, the plus equals += operator, and concat().

Note: + and += are known as assignment operators.

Let’s try all three!

The plus operator (+)

Let’s say you have two variables, called firstName and lastName, and you want assign the combined value of those variables to a third variable, called fullName. You can do that like this:

const firstName = "Steve"
const lastName = "Jobs"
const fullName = firstName + lastName

console.log(fullName) // SteveJobs

Add separation (space)

To make the fullName string value more readable, let’s add a space between Steve and Jobs. We’ll reuse same code from before, but this time we add a separator in between + '' +, like this:

const firstName = "Steve"
const lastName = "Jobs"
const fullName = firstName + " " + lastName

console.log(fullName) // Steve Jobs

Note: you could also add the separator by adding a space after "Steve " or before " Jobs" but that can lead to other formatting problems if you’re planning on using the firstName or lastName variables in a different context in your project. So I don’t recommend that approach.

The plus equals operator (+=)

If for whatever reason you don’t want to create a new variable (fullName) you can assign the lastName variable to firstName by using the plus equals operator +=:

console.log((firstName += lastName))
// SteveJobs

However, as you can see, now we need a separator again. To add multiple values with the += operator you might think this is possible:

firstName += " " += lastName

But that will throw you an error:

Uncaught SyntaxError: Invalid left-hand side in assignment

Instead you need to combine values on separate lines. First assign the space separator to firstName, and then assign the lastName to firstName afterward:

let firstName = "Steve"
let lastName = "Jobs"

// First assign separator value
firstName += " "
// Then assign last name
firstName += lastName
// Result: Steve Jobs

The concat() method

You can also join strings by using JavaScripts concat() method:

let firstName = "Steve"
let lastName = "Jobs"

const fullName = firstName.concat(lastName)

// SteveJobs

Once again, we need a separator. That’s straight forward with concat(). Just add a space with string and a comma, followed by the lastName variable:

const fullName = firstName.concat(" ", lastName)
// Steve Jobs

Which method should you use to join strings?

For performance reasons, Mozilla and many other credible JS sources strongly recommend using + or += whenever it’s possible.

As this JSPerf test case shows, string objects like concat() are way slower for the JavaScript interpreter (that runs in your browser) than string primitives like + and +=:

Performance chart from JSPerf showing that primitive strings perform much better than string objects.
It’s not even close. JSPerf source

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