Check if an Object has a Method in TypeScript

Check if an Object has a Method in TypeScript #

To check if an object has a method in TypeScript:

Mark the specific method as an optional key in the object's type.
Use typeof to check if accessing the method on the object returns a value with a function type.
If the typeof operator returns true, the method exists.

index.ts

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

const obj: Employee = {
  name: 'Alice',
  salary: 500,
};

if (typeof obj.getSalary === 'function') {
  console.log(obj.getSalary());
} else {
  console.log('getSalary method does not exist on object');
}

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

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

const obj: Employee = {
  name: 'Alice',
  salary: 500,
};

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

if (typeof obj.getSalary === 'function') {
  // 👇️ type of obj.getSalary is function here
  console.log(obj.getSalary());
} else {
  console.log('getSalary method does not exist on object');
}

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.

Here are some more examples of using the typeof operator.

index.js

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console.log(typeof {}); // 👉️ "object"
console.log(typeof []); // 👉️ "object"
console.log(typeof null); // 👉️ "object"
console.log(typeof (() => {})); // 👉️ "function"
console.log(typeof ''); // 👉️ "string"
console.log(typeof 0); // 👉️ "number"
console.log(typeof NaN); // 👉️ "number"
console.log(typeof undefined); // 👉️ "undefined"

Note that the return value from the operator is always wrapped in a string.

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Check if an Object has a Method in TypeScript

Check if an Object has a Method in TypeScript #

Borislav Hadzhiev