super Keyword in JavaScript – What Is the super Keyword?

The super keyword searches a parent class or object literal for a specified static or prototypal property.

For instance, consider the following snippet:

// Create a new class:
class Name {
  constructor() {
    console.log("Oluwatobi is my Name");
  }
}

// Create a child class:
class Bio extends Name {
  constructor() {
    // Use super to access the parent class's constructor:
    super();
  }
}

// Invoke the Bio constructor class:
new Bio();

// The invocation above will return:
// "Oluwatobi is my Name"
// {}

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The super() function call in the snippet above tells the computer to find a constructor in the parent class’s prototype chain.

You can use the super keyword as a “function caller” or “property accessor”. Let’s discuss how.

How to Use the super Keyword as a Function Caller

The super() function caller finds and invokes the parent class’s constructor() method.

In other words, super() allows you to access a parent class’s constructor from the constructor of a derived class.

Syntax of the super keyword as a function caller

super(argument1, …, argumentN);

Note

A super() function caller is valid only in a derived class’s constructor() method.

Example: How to use the super() function caller

// Create a new class:
class Name {
  constructor(name) {
    this.name = name;
  }
}

// Create a derived class:
class Bio extends Name {
  constructor(firstName) {
    // Use super to access the parent class's constructor:
    super(firstName);
  }
}

// Create a new object instance:
const myBio = new Bio("Oluwatobi");

// Check myBio's current value:
myBio;

// The invocation above will return:
// { name: "Oluwatobi" }

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The super() function call in the snippet above tells the computer to find and invoke the parent class’s constructor().

In other words, the super() function call searches for a constructor in Name’s prototype chain.

Note the following:

• Calling super() allows JavaScript to use the parent class’s constructor to initialize this. So, a super() function call is similar to writing this = new ParentClass().
• JavaScript requires you to call super() before using the keyword this. Otherwise, the browser will throw a ReferenceError. In other words, a derived class’s constructor cannot access an uninitialized keyword this.

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Example: What happens if you access this before super in a derived class’s constructor?

// Create a new class:
class Name {
  constructor(name) {
    this.name = name;
  }
}

// Create a derived class:
class Bio extends Name {
  constructor(firstName) {
    this.lastName = "Sofela"; // Using the keyword this before super will cause the browser to throw a ReferenceError:
    super(firstName);
  }
}

// Create a new object instance:
const myBio = new Bio("Oluwatobi");

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The snippet above throws an Uncaught ReferenceError because a derived class’s constructor cannot access the keyword this before the super() function caller.

Example: What happens if you use only this keyword in a derived class’s constructor?

// Create a new class:
class Name {
  createName() {
    return "Sofela";
  }
}

// Create a derived class:
class Bio extends Name {
  constructor() {
    this.firstName = "Oluwatobi"; // Using the keyword this before super will cause the browser to throw a ReferenceError:
  }
}

// Create a new object instance:
const myBio = new Bio();

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The snippet above throws an Uncaught ReferenceError because a derived class’s constructor cannot access the keyword this before the super() function caller.

Now that we know how to use the super keyword as a function caller, we can discuss using it as a property accessor.

How to Use the super Keyword as a Property Accessor

You can use the super keyword as a property accessor in your JavaScript classes and object literals.

• Class Usage: The super keyword searches a class’s parent for a specified static or prototypal class field. In other words, super allows you to access a parent class’s static or prototypal properties from a child class.
• Object Literal Usage: The super keyword searches an object’s parent for a specified prototypal property. In other words, super allows you to access a parent object’s prototypal properties from a child object.

Syntax of the super keyword as a dot notation property accessor

super.parentClassOrObjectProperty;

Example: Use the super keyword’s dot notation to access the parent class’s static field

// Create a new class:
class Name {
  // Create a static class field:
  static myName = "Oluwatobi";
}

// Create a derived class:
class Bio extends Name {
  // Create a static property from the parent class's static class field:
  static firstName = super.myName;
}

// Check firstName's current value:
Bio.firstName;

// The invocation above will return: "Oluwatobi"

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We used the super keyword in the snippet above to access the parent class’s static class field.

Note

Prefixing the firstName statement with a static keyword makes super find a myName static property in the parent class.

Suppose you omit the static keyword. In that case, super will search for a myName prototypal property in the parent class.

Example: Use the super keyword’s dot notation to access the parent class’s prototypal field

// Create a new class:
class Time {
  // Create a prototypal method:
  hourNow() {
    return new Date().getHours();
  }

  // Create a second prototypal method:
  minutesNow() {
    return new Date().getMinutes();
  }
}

// Create a derived class:
class Moment extends Time {
  // Create an instance property using the parent class's prototypal methods:
  currentMoment = `The time is ${super.hourNow()}:${super.minutesNow()}.`;
}

// Create a new object instance:
const momentNow = new Moment();

// Check momentNow's current value:
console.log(momentNow);

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We used the super keyword in the snippet above to access the parent class’s prototypal class fields.

Example: Use the super keyword’s dot notation to access a parent object’s prototypal property

// Create a new object:
const website = {
  // Create a method:
  showName() {
    return "CodeSweetly";
  },
};

// Create another object:
const company = {
  // Create a method:
  showCompany() {
    return super.showName();
  },
};

// Change company's dunder proto to the website object:
Object.setPrototypeOf(company, website);

// Invoke the showCompany method:
company.showCompany();

// The invocation above will return: "CodeSweetly"

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We used the super keyword in the snippet above to access the parent object’s showName() method.

Note

The Object.setPrototypeOf() code changes the company’s [[Prototype]] property to the website object. Therefore, the company object’s prototype chain will look like this:

{ showCompany: showCompany() } ---> { showName: showName() } ---> Object.prototype ---> null

You can also use the super keyword as a bracket notation property accessor to search a parent class or object literal for a specified static or prototypal property.

Syntax of the super keyword as a bracket notation property accessor

super[expresssion];

Example: Use the super keyword’s bracket notation to access a parent class’s static field

// Create a new class:
class Name {
  // Create a static class field:
  static myName = "Oluwatobi";
}

// Create a derived class:
class Bio extends Name {
  // Create a static property from the parent class's static class field:
  static firstName = super["myName"];
}

// Check firstName's current value:
Bio.firstName;

// The invocation above will return: "Oluwatobi"

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We used the super keyword in the snippet above to access the parent class’s static class field.

Note: super cannot access a parent class’s instance class field because JavaScript sets an instance property on the object instance, not the class itself or its prototype chain. (super searches only for a parent’s static or prototypal properties.)

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Example: Use the super keyword to access the parent class’s instance field

// Create a new class:
class Name {
  // Create an instance class field:
  myName = "Oluwatobi";
}

// Create a derived class:
class Bio extends Name {
  // Create an instance property from the parent class's instance class field:
  firstName = super.myName;
}

// Create a new object instance:
const myBio = new Bio();

// Check myBio's current value:
myBio;

// The invocation above will return:
// { myName: "Oluwatobi", firstName: undefined }

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The firstName property’s value is undefined because super could not find a prototypal myName field on the parent class.

Note: The keywords super and this allow you to search for a specified property in an object’s prototype chain. But they work in different ways. Let’s discuss their differences now.

super vs. this Keyword: What’s the Difference?

The difference between the super and this keyword is as follows:

• super searches for a specified prototypal property in a parent class’s prototype chain.
• this searches for a specified prototypal property from a class’s object instance’s own properties to its prototype chain.

In other words, super starts its search from the parent class’s prototype property. But this searches from an object instance’s local scope to its prototype chain.

For instance, consider the following code:

// Create a new class:
class ParentClass {
  // Create a prototypal method:
  showId() {
    return "I am a parent.";
  }
}

// Create a derived class:
class ChildClass extends ParentClass {
  // Create a prototypal method:
  showId() {
    return "I am a child.";
  }
  // Create another prototypal method:
  getId() {
    console.log(super.showId());
    console.log(this.showId());
  }
}

// Create a new object instance:
const instanceObject = new ChildClass();

// Invoke the instanceObject's getId() method:
instanceObject.getId();

// The invocation above will return:
// "I am a parent."
// "I am a child."

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Here’s how super and this performed their searches:

	super	this
1.	Find showId() in ParentClass’s prototype chain, starting from ParentClass.prototype. Found it.	Find showId() in instanceObject’s own properties. Found none.
2.	(Suppose showId() is not in ParentClass.prototype. In that case, super will continue its search in Object.prototype.)	Find showId() in instanceObject’s prototype chain, starting from ChildClass.prototype. Found it.
3.		(Suppose showId() is not in ChildClass.prototype. In that case, this will continue its search in ParentClass.prototype.)
4.		(Suppose showId() is not in ChildClass.prototype and ParentClass.prototype. In that case, this will continue its search in Object.prototype.)

You can see that super shortens the steps required to find a prototypal method.