How can make type optional in typescript

Question

How can I make some type optional in Typescript?

I have following code:

const A = <T>(value: T) => new Clazz(value);
const B = <U>(value: U) => new Clazz(undefined, value);

class Clazz<T, U> {
  constructor(private a?: T, private b?: U) {}

  public map<Z>(callback: (value: T) => Z): Clazz<Z, U> {
    return this.a 
      ? A(callback(this.a)) 
      : B(this.b);
  } 
}

But this code fails with error:

Type 'Clazz<Z, {}> | Clazz<undefined, U | undefined>' is not assignable to type 'Clazz<Z, U>'. 
Type 'Clazz<Z, {}>' is not assignable to type 'Clazz<Z, U>'. 
Type '{}' is not assignable to type 'U'.

What is the best way to solve this problem?

My tsconfig.json looks like that:

{
  "compilerOptions": {
    "baseUrl": "",
    "declaration": true,
    "lib": ["es6", "dom"],
    "mapRoot": "./src",
    "module": "es2015",
    "moduleResolution": "node",
    "noEmitHelpers": true,
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "outDir": "./dist",
    "sourceMap": true,
    "strictNullChecks": true,
    "suppressImplicitAnyIndexErrors": true,
    "target": "es2015",
    "typeRoots": [
      "./node_modules/@types"
    ]
  }
}

GPicazo · Accepted Answer · 2017-01-26 20:19:40Z

1

The problem is that the return type of the map function does not match the return type of method A because A does not provide the expected contructor parameters for the Clazz class. You can use the same work-around that you use for method B, which is to pass an undefined or null value for the second parameter of the Clazz constructor as so:

const A = <T>(value: T) => new Clazz(value, null);
const B = <U>(value: U) => new Clazz(null, value);

class Clazz<T, U> {
  constructor(private a?: T, private b?: U|null) {}

  public map<Z>(callback: (value: T|undefined) => Z): Clazz<Z|null, U> {
    return this.a 
      ? A(callback(this.a)) 
      : B(this.b);
  } 
}

edited Jan 26, 2017 at 20:19

answered Jan 26, 2017 at 16:49

GPicazo

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3 Comments

Lodin Over a year ago

Thanks, it did the trick. However, to fulfil the strict requirements of my tsconfig.json (I have added it just now), all generic types (in A and B functions, T and U in Clazz and Z in map) have to extend undefined type. If you update the answer, I'll be able to mark it as correct.

GPicazo Over a year ago

I believe in that case you can use null instead of undefined.

Lodin Over a year ago

hmm, I've just checked it, and there is some problems with B(this.b) line. It says that Argument of type 'U | undefined' is not assignable to parameter of type 'null'.

GPicazo · Accepted Answer · 2017-01-26 20:18:29Z

I must admit I am having a hard time understanding what you are doing there and why - an explanation would be great.

One way to solve your problem would be to parametrize your code like this

const A = <T>(value: T) => new Clazz(value, null);
const B = <U>(value: U) => new Clazz(null, value);

class Clazz<T, U> {
  constructor(private a?: T, private b?: U|null) {}

  public map<Z>(callback: (value: T|undefined) => Z): Clazz<Z|null, U> {
    return this.a 
      ? A(callback(this.a)) 
      : B(this.b);
  } 
}

This compiles but I have a hard time to even express what this map does

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