Check if Object is instance of Class in TypeScript

Check if Object is instance of Class in TypeScript #

Use the instanceof operator to check if an object is an instance of a class, e.g. if (myObj instanceof MyClass) {}. The instanceof operator checks if the prototype property of the constructor appears in the prototype chain of the object and returns true if it does.

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class Person {}

const p1 = new Person();

if (p1 instanceof Person) {
  console.log('✅ is instance of Person');
} else {
  console.log('⛔️ is not instance of Person');
}

class Animal {}
console.log(p1 instanceof Animal); // 👉️ false

The instanceof operator returns a boolean value indicating whether the prototype property of the constructor appears in the prototype chain of the object.

The p1 object was created using the Person class, so it is an instance of the class.

The instanceof operator is very useful in TypeScript because it can be used as a type guard.

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class Person {
  walk() {
    console.log('person is walking');
  }
}
class Animal {
  run() {
    console.log('animal is running');
  }
}

function example(x: Person | Animal) {
  // 👉️ x is type Person or Animal here
  if (x instanceof Person) {
    // 👇️ x is type Person here
    x.walk();
  } else {
    // 👇️ x is type Animal here
    x.run();
  }
}

The function takes a parameter of type Person or Animal, so before we access a class-specific method, we have to check an instance of which class was passed to the function.

If you access the constructor.name property, you can see that the class name of the p1 object is Person.

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class Person {}

const p1 = new Person();

console.log(p1.constructor.name); // 👉️ Person

We accessed the name property on the Object.constructor property.

The Object.constructor property returns a reference to the constructor function, from which the object was created.

The instanceof operator checks for the presence of constructor.prototype in the object's prototype chain.

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class Person {}

const p1 = new Person();

// 👇️ true
console.log(Object.getPrototypeOf(p1) === Person.prototype);

Note that checking if an object is not an instance of a class is a bit tricky.

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class Person {}
class Animal {}

const person = new Person();

if (!(person instanceof Animal)) {
  console.log('person is NOT an instance of Animal');
}

Notice that we used parenthesis around the instanceof check before negating it.

A commonly seen mistake is to omit the parenthesis, e.g.:

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class Person {}
class Animal {}

const person = new Person();

// 👇️ Don't do this

// ⛔️ Error: The left-hand side of an 'instanceof'
// expression must be of type 'any', an object type
// or a type parameter.ts(2358)
if (!person instanceof Animal) {
  console.log('person is NOT an instance of Animal');
}

This code example inverts the value of the object and converts it to boolean, so it becomes false. We then check if false is an instance of the Animal class.

Wrapping the expression in parenthesis allows us to evaluate it as a whole.

If you don't like the parenthesis approach you can explicitly check if the instanceof operator returns false.

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class Person {}
class Animal {}

const person = new Person();

if (person instanceof Animal === false) {
  console.log('person is NOT an instance of Animal');
}

This achieves the same goal as using the logical NOT (!) operator with parenthesis, but is a little easier to read.