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  3. JavaScript — What Is `Symbol.toStringTag` and How to Use It

JavaScript — What Is `Symbol.toStringTag` and How to Use It

You may have seen JavaScript printing [object Object] when casting an object to a string. The [object Object] notation doesn’t give you information about the kind of object. You have to guess what object could cause this printing.

This is where Symbol.toStringTag is helpful and this tutorial shows you how to use it in JavaScript.

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What is Symbol.toStringTag?

JavaScript’s default behavior when casting an object to a string is printing the type and related constructor. You can customize the string output by implementing a toString method. But you don’t always want to bring your handling for string conversions because your object only carries data.

Another option for more visibility in your application is to use the Symbol.toStringTag. This static property represents the well-known @@toStringTag symbol. JavaScript looks up this symbol when calling the Object.prototype.toString() method on an object.

The Symbol.toStringTag property is not writable, not enumerable, and not configurable. Most built-in JavaScript objects have their own @@toStringTag property. Their output then contains the related constructor class:

Object.prototype.toString.call({})  
// '[object Object]'

Object.prototype.toString.call([])  
// '[object Array]'

Object.prototype.toString.call(new Map())  
// '[object Map]'

Object.prototype.toString.call(Promise.resolve())  
// '[object Promise]'

Your custom class uses the default “Object” tag:

class User {}

Object.prototype.toString.call(new User())  
// "[object Object]"

The next section shows you how to implement the toStringTag in your class.

Using Symbol.toStringTag in JavaScript

You can implement the @@toStringTag symbol as a property on your objects and classes. The property must return a string that JavaScript then uses when casting the object to a string value.

Here’s the sample implementation on a User class:

class User {  
  get [Symbol.toStringTag]() {
    return this.constructor.name
  }
}

Object.prototype.toString.call(new User())  
// "[object User]"

You can also use the Symbol.toStringTag on objects:

const user = {  
  name: 'Marcus',

  get [Symbol.toStringTag] () {
    return 'UserObject'
  }
}

console.log(Object.prototype.toString.call(user))  
// "[object UserObject]"

That's it!

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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