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Introduction to JavaScript Property Getters and Setters (Accessor Properties)

alexdevero profile image Alex Devero Originally published at blog.alexdevero.com ・9 min read

Property getters and setters allow you to change the default behavior when accessing or modifying object properties. This tutorial teach you all you need to know about them. You will learn what JavaScript property getters and setters are, how they work and how to use them.

Properties and property getters and setters

In JavaScript, there are two types of properties. The first type is data properties. These are the properties you usually use when you work with objects. The second type is called "accessor properties". These are a bit different. Put simply, accessor properties are methods.

These methods are executed every time you work with a property. When you access, or get, a value or when you set or change a value. What about property getters and setters? These two represent this group of properties, the accessor properties. To be more specific, getter is that function that is executed when you access some value.

Setter, on the other hand, is that function that is executed when you set or change a value. Interesting thing is that these methods are executed automatically. Well, this assumes there is some existing getter or setter. You don't have to call them explicitly. You don't have to call them at all.

All that is needed is someone trying to access and set a value of some property. If there is a getter or setter for that specific property it will be executed. Now, let's take a look at each.

Property getters

Property getters, or methods, are used to access properties of objects. When you want to get value of some property, and this property has a getter method, that method will be executed. The getter method looks like a regular object method. The difference is the get keyword.

This get keyword is what tells JavaScript that you don't want to create regular object method, but a getter method. The way to use this keyword is to put it as first, before the name of the getter method. What follows is the name of the getter, parentheses and function body.

// Syntax
// Create an object
const myObj = {
  // Example of a getter method
  // this getter will be executed
  // when you use myObj.myGetter
  get myGetter() {
    // Return something
    return ''
  }
}

// Execute the getter method
// NOTE: when you use getter method
// don't use parentheses at the end
myObj.myGetter


// Example:
// Create an object
const dog = {
  name: 'Jack',

  // Create getter method
  // this getter will be executed
  // when you use dog.getName
  get getName() {
    // this here refers to the "dog" object
    return `My dog's name is: ${this.name}`
  }
}

// Execute the getter method
console.log(dog.getName)
// Output:
// "My dog's name is: Jack"
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One thing to remember. Property getter should always return something, some value. If it doesn't return anything, you will get undefined when you try to use the getter. So, if you do add getter method, make sure it also contains return statement and returns something.

// Create an object
const dog = {
  name: 'Jack',

  get getName() {}
}

// Execute the getter method "getName"
console.log(dog.getName)
// Output:
// undefined
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Property setters

When you set or change value of some property JavaScript will execute existing property setter, or setter method, for that property. The syntax of setter method is almost the same as of getter. One thing that is different is the keyword. When you want to define a setter you have to use set keyword, instead of get.

This keyword tells JavaScript that the method that follows is a setter method. Another thing that is different is that you will probably want to specify at least one parameter. The setter method is used to set value. Method parameter is a way to pass that value to a setter so it can be used.

Lastly, unlike, the getter method, the setter method doesn't have to return anything. Setter is used to set values and no one probably expects it to return anything. So, omitting return statement is perfectly fine.

// Syntax
// Create an object
const myObj = {
  // Example of a setter method
  // this setter will be executed
  // when you use myObj.mySetter = ...
  get mySetter() {
    // Return something
    return ''
  }
}

// Execute the setter method "mySetter"
// NOTE: when you use setter method
// you use as if you were assigning a value
myObj.mySetter = 'Hello'


// Example:
// Create an object
const user = {
  name: 'Stuart Douglass',
  isAdmin: false,

  // Create setter method
  // this setter will be executed
  // when you use user.setName = ...
  set setName(newName) {
    // Allow only string with more than 0 characters
    if (typeof newName === 'string' && newName.length > 0) {
      this.name = newName
    } else {
      if (typeof newName !== 'string') {
        console.log('Please use only string.')
      } else if (newName.length === 0) {
        console.log('Please use name with more than 0 characters.')
      }
    }
  }
}

// Try to change the value of "name" to an empty string
// This executes the setter method for "name"
user.setName = ''
// 'Please use name with more than 0 characters.'

// Try to change the value of "name" to a number
// This executes the setter method for "name"
user.setName = 55
// 'Please use only string.'

// Check the value of "name" property
// This executes the getter method for "name"
console.log(user.name)
// Output:
// 'Stuart Douglass'

// Try to change the value of "name" to a string
// This executes the setter method for "name"
user.setName = 'Jeremy Guire'

// Check the value of "name" property again
// This executes the getter method for "name"
console.log(user.name)
// Output:
// 'Jeremy Guire'
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Getters and setters as property wrappers

As you saw on previous examples, you can use getters and setters to restrict changes to property values. For example, you can reject change of a string value if new value is not a string. Or, you can reject the change if the new string is empty. You saw this in the previous with setName setter method.

In this example, there is an if...else statement that checks the new value before it lets it override the old value. This is one potential use case for getter and setter methods. You can use these methods to dynamically check values before someone can access them and/or change them.

One problem is that you can't create getter or setter with the same name as existing property. This unfortunately doesn't work. However, you do something different. You can change the name of those original properties to maybe less user-friendly. Then, you can use user-friendly names for getter and setter methods.

In programming, there is a well-known convention to start property name with an underscore (_) to mark it as internal. Internal means that no one should use it directly from the outside the object. You can use this convention with properties for which you want to add getters and setters.

So, here is what to do. First, prefix all properties that will have getter and setter methods with underscore. Second, create property getters and setters with the same names, but now without the underscore prefix. This will give you properties with friendly names that you have more control over.

const car = {
  // Add properties, prefixed with '_'
  _manufacturer: 'BWM',
  _model: 'i8',
  _year: '2020',

  // Create getter method for "_manufacturer"
  get manufacturer() {
    return this._manufacturer
  },

  // Create setter method for "_manufacturer"
  set manufacturer(newManufacturer) {
    if (typeof newManufacturer === 'string' && newManufacturer.length > 0) {
      this._manufacturer = newManufacturer
    }
  },

  // Create getter method for "_model"
  get model() {
    return this._model
  },

  // Create setter method for "_model"
  set model(newModel) {
    if (typeof newModel === 'string' && newModel.length > 0) {
      this._model = newModel
    }
  },

  // Create getter method for "_year"
  get year() {
    return this._year
  },

  // Create setter method for "_year"
  set year(newYear) {
    if (typeof newYear === 'string' && newYear.length > 0) {
      this._year = newYear
    }
  }
}

// Get current manufacturer
// Execute getter methods
console.log(car.manufacturer)
// Output:
// 'BWM'

// Get current model
console.log(car.model)
// Output:
// 'i8'

// Get current year
console.log(car.year)
// Output:
// '2020'

// Change some values
// Execute setter methods
car.manufacturer = 'Tesla'
car.model = 'Model S'

// Get new manufacturer
// Execute getter methods
console.log(car.manufacturer)
// Output:
// 'Tesla'

// Get new model
console.log(car.model)
// Output:
// 'Model S'
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Creating getters and setters on the go

So far, we've looked only on creating property getters and setters at the time of creating an object. However, there is a way to add getter and setter methods also to an object that already exists. You can do this with the help of Object.defineProperty() method. This method allows you to add new properties to objects or change existing.

You can use this method also to add or change accessor properties, property getters and setters. Adding getters and setters with this method is similar to adding them when you create an object. When you use the defineProperty() you pass in three parameters. The first argument is the object you want to update.

The second parameter is the property you want to add or change. In case of property getters and setters, it is the property for which you want to add getter and/or setter method. For new property, the last parameter is for object with descriptors, such as enumerable, configurable, value and so on.

In case of getters and setters, you replace the object with descriptors with an object containing the getter and setter method. The syntax for both, getter and setter, is in this case almost the same as in previous examples. One difference is missing method name. You specified that as the second parameter, the property name.

// Create an object
const book = {
  _title: 'Six of Crows',
  _author: 'Leigh Bardugo',
  _pubDate: 'February 6, 2018'
}

// Add getter and setter for title
// Parameter 1: object to update
// Parameter 2: property to add/update
// Parameter 3: object containing getter and setter
Object.defineProperty(book, 'title', {
  get() {
    return this._title
  },
  set(newTitle) {
    if (typeof newTitle === 'string' && newTitle.length > 0) {
      this._title = newTitle
    }
  }
})

// Add getter and setter for title
// Parameter 1: object to update
// Parameter 2: property to add/update
// Parameter 3: object containing getter and setter
Object.defineProperty(book, 'author', {
  get() {
    return this._author
  },
  set(newAuthor) {
    if (typeof newAuthor === 'string' && newAuthor.length > 0) {
      this._author = newAuthor
    }
  }
})

// Add getter and setter for title
// Parameter 1: object to update
// Parameter 2: property to add/update
// Parameter 3: object containing getter and setter
Object.defineProperty(book, 'pubDate', {
  get() {
    return this._pubDate
  },
  set(newPubDate) {
    if (typeof newPubDate === 'string' && newPubDate.length > 0) {
      this._pubDate = newPubDate
    }
  }
})

// Get current book title
// This executes the getter method for "title"
console.log(book.title)
// Output:
// 'Six of Crows'

// Get current book author
// This executes the getter method for "author"
console.log(book.author)
// Output:
// 'Leigh Bardugo'

// Get current book publication date
// This executes the getter method for "pubDate"
console.log(book.pubDate)
// Output:
// 'February 6, 2018'

// Change book data
// This executes the setter method for "title"
book.title = 'Red Rising'
// This executes the setter method for "author"
book.author = 'Pierce Brown'
// This executes the setter method for "pubDate"
book.pubDate = 'January 28, 2014'

// Get new book title
// This executes the getter method for "title" again
console.log(book.title)
// Output:
// 'Red Rising'

// Get new book author
// This executes the getter method for "author" again
console.log(book.author)
// Output:
// 'Pierce Brown'

// Get new book publication date
// This executes the getter method for "pubDate" again
console.log(book.pubDate)
// Output:
// 'January 28, 2014'
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Getters and setters the old way

The get and set keywords were introduced in JavaScript in ES5. Before this, it was possible to create property getters and setters by using regular object methods. So, if you want to create getter and setter methods the old way, you can. You can use either syntax with the function keyword or ES2015 syntax that is without it.

If you use regular methods to create property getters and setters remember one thing. You also have to use these getter and setter methods as methods. This means calling them as object methods. Also, in case of the setter method, you have to pass the new value as an argument to that setter method.

// Using syntax without function keyword (ES2015 syntax)
// Create an object
const person = {
  // Add some properties
  // Let's use the '_' convention
  // for internal properties
  _name: 'Jack Doe',
  _status: 'online',

  // Add getter method for "name"
  getName() {
    return this._name
  },

  // Add setter method for "name"
  setName(newName) {
    if (typeof newName === 'string' && newName.length > 0) {
      this._name = newName
    }
  }
}

// Use getter method to get current name
// NOTE: getter is now a regular method
// so you have to call it, as a method
person.getName()
// Output:
// 'Jack Doe'

// Use setter method to change the name
// NOTE: setter is also a regular method
// so you have to call it as a method
// and pass new value as an argument
person.setName('Stuart Mill')

// Use getter method to get the new name
person.getName()
// Output:
// 'Stuart Mill'


// Using syntax with function keyword (pre-ES2015 syntax)
const person = {
  _name: 'Jack Doe',
  _status: 'online',

  // Getter method with function keyword
  getName: function() {
    return this._name
  },

  // Setter method with function keyword
  setName: function(newName) {
    if (typeof newName === 'string' && newName.length > 0) {
      this._name = newName
    }
  }
}
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Conclusion: Introduction to JavaScript property getters and setters

Property getters and setters can be quite useful. You can use them to change the behavior for accessing property, and to get more control over how these values can be changed. Especially if you use them as property wrappers and keep those properties internal. I hope that this tutorial helped you understand what property getters and setters are, how they work and how to use them.

Discussion

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sametweb profile image
Samet Mutevelli

Thanks for sharing. I recently learned about getters/setters. While it is a good concept in terms of OOP, I wonder if there is a use case in the React ecosystem. I would like to hear your thoughts on that.

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unsignedmind profile image
unsignedmind

First: It's good to know how it works under the hood but today you can just use es6 classes and with typescript you dont need to check the type yourself. Just define the input a string.
Second: Whenever you need a a combination of values a getter is usefull. For example a component has 2 attributes. One const for MAX_ENTRIES and another one for an array entries. The getter would be smth like entriesLeft and returns MAX_ENTRIES - entries.length.