Is there a difference between a variable with a ? or undefined type in Typescript

user_13179199 0
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In Typescript there are two different methods of saying that a property can be undefined when creating an interface:

Method 1:

interface m1 {
    prop?: randomType; // type is (randomType | undefined)
}

Method 2:

interface m2 {
    prop: randomType|undefined; // type is (randomType | undefined)
}

The only immediately apparent between these two that I have noticed is that when hovering over prop IntelliSense will show undefined in blue for Method 1 and green for Method 2.

Are there any other differences between these?

typescript undefined
1 Answers

To make it simple : with ? the field becomes optional :

type randomType = {}

interface M1 {
  prop?: randomType; // type is (randomType | undefined)
}

interface M2 {
  prop: randomType | undefined; // type is (randomType | undefined)
}

const m1: M1 = {} // OK
const m2: M2 = {} // NOK
const m22: M2 = { prop: undefined } // OK

Playground

user_8841230
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