What is the utility of **typescript optional** properties? I'm new to TypeScript and would like to understand the difference between them

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1

What is the utility of typescript optional properties? I’m new to TypeScript and would like to understand the difference between the following declarations:

  1. a?: number;
  2. a: number | undefined;

How do they behave differently in TypeScript?

2

Hi

I would love to help you here mate.

Using a?: number; for Optional Property in TypeScript: In TypeScript, when you declare a property as a?: number;, it signifies that the property is optional. This means the property can either have a number value or be completely absent (i.e., undefined). The property is not required when creating an object, and the TypeScript compiler does not enforce its presence. Example:

interface MyObject {
    a?: number; // optional property
}
const obj: MyObject = {};

// No error, a is optional

3

Using a: number | undefined; for Explicitly Undefined Property in TypeScript: The declaration a: number | undefined; means that the property a must always be present, but its value can either be a number or explicitly undefined.

his is different from a?: number;, as the property must always be defined, even if it’s set to undefined. Example:

interface MyObject {
    a: number | undefined; // required property that can be undefined
}

const obj: MyObject = { a: undefined }; // No error, but a cannot be omitted

4

Just to give you clarity on when to use what and which

When to Use a?: number; vs a: number | undefined;: If you want to indicate that a property is completely optional and may be missing from an object, use a?: number;. If you want to ensure the property always exists but it can explicitly be undefined, use a: number | undefined;.

The key difference is that the a?: number allows the property to be omitted entirely, while the a: number | undefined ensures the property is always present but can be set to undefined. Example:

interface MyObject {
    a?: number; // a is optional
    b: number | undefined; // b is required but can be undefined
}
const obj1: MyObject = {};  // Valid for a but not for b
const obj2: MyObject = { b: undefined };  // Valid, b is required but undefined