CODEWITHSUNDEEP
typescript
Caleb McHenry
Caleb McHenry

Reputation: 11

Semantics of optional params in typescript

As a user of some else's code I want to be able to look a function and figure out what is required based on if the type is optional.

For example:

function doSomething(required: string, optional?: string)

But as a dev who is writing code for someone else to use I want to still signify that a param is required but I want type checking as if it wasn't required since I can't guarantee my end user is also using typescript and I want to handle bad arguments gracefully.

Is it possible to have my cake and eat it too? Can I signify a param as not optional but still get compile errors if I don't check or handle the argument being undefined?

Is there a semantic difference between ?: string and : string | undefined

Or is the solution to just have preconditions on all required fields?

Upvotes: 1

Views: 100

Answers (2)

Evgeny Klimenchenko
Evgeny Klimenchenko

Reputation: 1194

If it is a matter to remember to handle undefined cases even though the value required then you can recast the passed paramters and the typescript will yell at you at compile time for not checking if this recasted value is undefined

here is an example where I used as string | undefined inside function, for user of the function they will see that all parameters are required, but you still will be defensive.

  function doSomthingElse(requiredString1: string, requiredString2: string) {}

  function doSomething(required: string, required2: string) {
    const recasted = required2 as string | undefined

    doSomthingElse(required, recasted) //TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string
  }

Upvotes: 0

Titian Cernicova-Dragomir
Titian Cernicova-Dragomir

Reputation: 250156

An optional parameter can be omitted in the call. A required parameter typed as T | undefined must still be present, although you can pass in undefined

function doSomething(required: string, optional: string | undefined) {

}

doSomething("") // error in TS
doSomething("", undefined)

Playground Link

If the user does not use Typescript none of this really matters, as the user can pass in fewer parameters anyway.

function doSomething(required: string, optional: string) {

}

doSomething("") /// Bad in ts, but fine in JS

Playground Link

Upvotes: 2

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