Check if a Method exists in a Class in TypeScript

Check if a Method exists in a Class in TypeScript #

Use the typeof operator to check if a method exists in a class, e.g. if (typeof myInstance.myMethod === 'function') {}. The typeof operator returns a string that indicates the type of the specific value and will return function if the method exists in the class.

index.ts

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class Employee {
  constructor(public name: string, public salary: number) {
    this.name = name;
    this.salary = salary;
  }
}

interface Employee {
  getSalary?: () => number;
}

Employee.prototype.getSalary = function () {
  return this.salary;
};

const emp1 = new Employee('James', 100);

// ✅ Check for method's existence
if (typeof emp1.getSalary === 'function') {
  console.log(emp1.getSalary()); // 👉️ 100
} else {
  console.log('method does NOT exist in the class');
}

Note that we used a question mark to set the getSalary method to optional in the Employee interface.

This means that the getSalary property can either have a value of undefined or be a method that returns a number.

We used the typeof operator to check if the getSalary property is a method.

index.ts

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console.log(typeof function () {}); // 👉️ "function"

The typeof operator returns a string that indicates the type of the specific value.

This serves as a type guard in TypeScript.

index.ts

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class Employee {
  constructor(public name: string, public salary: number) {
    this.name = name;
    this.salary = salary;
  }
}

interface Employee {
  getSalary?: () => number;
}

Employee.prototype.getSalary = function () {
  return this.salary;
};

const emp1 = new Employee('James', 100);

// 👇️ type is function or undefined
emp1.getSalary;

if (typeof emp1.getSalary === 'function') {
  // 👇️ type of emp1.getSalary is function
  console.log(emp1.getSalary()); // 👉️ 100
} else {
  console.log('method does NOT exist in the class');
}

The getSalary property is optional, so it can either have a value of undefined or be a function.

In our if statement we check if the specific property has a type of function, so TypeScript knows that it can't possibly be undefined in the if block.

Check if a Method exists in a Class in TypeScript

Check if a Method exists in a Class in TypeScript #

Borislav Hadzhiev