Check if Value with Unknown Type contains Property in TS

# Check if Value with Unknown Type contains Property in TS

Use a user-defined type guard to check if a value with unknown type contains a property in TypeScript.

The user-defined type guard consists of a function that checks if the specific property is contained in the object and returns a predicate.

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interface Person {
  name: string;
  age: string;
}

function isPerson(obj: unknown): obj is Person {
  return (
    typeof obj === 'object' && obj !== null && 'name' in obj && 'age' in obj
  );
}

const obj: unknown = {
  name: 'Bobby Hadz',
  age: 30,
};

if (isPerson(obj)) {
  console.log(obj.name); // 👉️ "Bobby Hadz"
  console.log(obj.age); // 👉️ 30
}

We used a user-defined type guard to check if an object contains the name and age properties.

We first check if the passed-in value has a type of object.

Unfortunately, we also have to check that the object isn't equal to null because null has a type of object in JavaScript (and TypeScript).

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console.log(typeof null); // 👉️ "object"

At this point, we can use the in operator to check if the name and age properties are contained in the object.

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const obj = {
  name: 'Bobby Hadz',
  age: 29,
};

console.log('name' in obj); // 👉️ true
console.log('age' in obj); // 👉️ true

console.log('test' in obj); // 👉️ false

The in operator checks if a specific property is contained in the object or its prototype chain and returns a boolean result - true if it is and false otherwise.

User-defined type guards are useful in this situation because in the if block TypeScript types the object to be of the specified type.

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interface Person {
  name: string;
  age: string;
}

function isPerson(obj: unknown): obj is Person {
  return (
    typeof obj === 'object' && obj !== null && 'name' in obj && 'age' in obj
  );
}

const obj: unknown = {
  name: 'Bobby Hadz',
  age: 30,
};

if (isPerson(obj)) {
  // 👉️ obj is of type Person here
  console.log(obj.name); // 👉️ "Bobby Hadz"
  console.log(obj.age); // 👉️ 30
}

Once we are in the if block, TypeScript knows the obj variable has a type of Person and allows us to access all of the properties a Person has.

If you get the error that Object is of type 'unknown', click on the link and follow the instructions.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Type 'unknown' is not assignable to type in TypeScript
• Catch clause variable type annotation must be any or unknown if specified