To avoid undefined values when using or accessing the optional object properties, the basic idea is to check the property value using an if conditional statement or the optional chaining operator before accessing the object property in TypeScript.

TL;DR

// object type declaration
let personDetails: {
  name: string;
  age?: number;
};

// object
personDetails = {
  name: "John Doe",
};

// Solution 1: ✅
// check if the 'age' property exists in the
// 'personDetails' object before accessing it
// using an if statement conditional check
if (personDetails.age) {
  const age = personDetails.age;
}

// Solution 2: ✅
// check if the 'age' property exists in the
// 'personDetails' object before accessing it
// using optional chaining operator in TypeScript
const age = personDetails?.age || "No age value is supplied";

To understand it better let's make an object called personDetails with 2 properties called name (string type) and age (number type) where the age property is an optional property. it can be done like this,

// object type declaration
let personDetails: {
  name: string;
  age?: number;
};

// object
personDetails = {
  name: "John Doe",
};

Now the object personDetails is valid and is allowed to be executed by the TypeScript compiler since the age property is optional.

Now let's try to access the value of the age property before the value is set.

It can be done like this,

// object type declaration
let personDetails: {
  name: string;
  age?: number;
};

// object
personDetails = {
  name: "John Doe",
};

// try to access the 'age' property
// in the personDetails object
const age = personDetails.age; // 🤯 undefined

As you can see from the above code, the value of the age property is undefined because the value is not set which may crash our program during the runtime or results in invalid values.

Typescript allowed us to access the age property since it is an optional property and Typescript assumed that we know what we are doing.

Now to avoid this issue we can either use conditional check using an if statement or use the optional chaining operator like this,

// object type declaration
let personDetails: {
  name: string;
  age?: number;
};

// object
personDetails = {
  name: "John Doe",
};

// Solution 1: ✅
// check if the 'age' property exists in the
// 'personDetails' object before accessing it
// using an if statement conditional check
if (personDetails.age) {
  const age = personDetails.age;
}

// Solution 2: ✅
// check if the 'age' property exists in the
// 'personDetails' object before accessing it
// using optional chaining operator in TypeScript
const age = personDetails?.age || "No age value is supplied";

As you can see from the above code we have corrected our code from being resulting in undefined values if the age property is not set.

See the execution of the above code live in codesandbox.

That's all 😃!

Feel free to share if you found this useful 😃.

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