Writing for: Either attribute "a" is defined or attributes "b" and "c" are both defined

Question

Writing for: Either attribute "a" is defined or attributes "b" and "c" are both defined

How can TypeScript understand whether this.a will be set or if this.b and this.c will be set?

In the given code, it is ensured that there will never be a situation where all items are set or none are set due to how the constructor is structured.

export class MyClass {

  readonly a?: SomeClass
  readonly b?: string
  readonly c?: string

  constructor(a: SomeClass)
  constructor(b: string, c: string)
  constructor(...args: ([SomeClass] | [string, string])) {
    if(args.length === 1) {
      this.a = args[0]
    } else {
      this.b = args[0]
      this.c = args[1]
    }
  }

  echo() {
    if (this.a instanceof SomeClass) {
    } else {
       someFunction(this.b, this.c)
    }
  }

}

someFunction is showing an error:

Argument of type 'string | undefined' is not assignable to parameter of type 'string'.

I am aware of using

else if(typeof this.b === 'string' && typeof this.c === 'string')

, but I'm looking for a more concise approach. Is there another way to solve this?

Example

typescript

Answer 1

Answer №1

Either the property "a" is defined or both properties "b" and "c" are defined

The main idea here revolves around the concept of "or". Utilize a TypeScript union type to achieve this, such as {a} | {b,c}. Below is a comprehensive example:

class SomeClass { };

export class MyClass {

    readonly values: { a: SomeClass } | { b: string, c: string }

    constructor(a: SomeClass)
    constructor(b: string, c: string)
    constructor(...args: ([SomeClass] | [string, string])) {
        if (args.length === 1) {
            this.values = { a: args[0] };
        } else {
            this.values = { b: args[0], c: args[1] };
        }
    }

    echo() {
        if ('a' in this.values) {
            const { a } = this.values;
        } else {
            const { b, c } = this.values;
        }
    }
}

Answer 2

Either the property "a" is defined or both properties "b" and "c" are defined

The main idea here revolves around the concept of "or". Utilize a TypeScript union type to achieve this, such as {a} | {b,c}. Below is a comprehensive example:

class SomeClass { };

export class MyClass {

    readonly values: { a: SomeClass } | { b: string, c: string }

    constructor(a: SomeClass)
    constructor(b: string, c: string)
    constructor(...args: ([SomeClass] | [string, string])) {
        if (args.length === 1) {
            this.values = { a: args[0] };
        } else {
            this.values = { b: args[0], c: args[1] };
        }
    }

    echo() {
        if ('a' in this.values) {
            const { a } = this.values;
        } else {
            const { b, c } = this.values;
        }
    }
}

Answer 3

Answer №2

//if this.a is not defined, then execute someFunction with this.b and this.c
if (!this.a) {
    someFunction(this.b!, this.c!)    
}

// alternatively, only call someFunction if both this.b and this.c are assigned strings
if(this.b && this.c){
    someFunction(this.b, this.c)
}

Link to Interactive Playground

Answer 4

//if this.a is not defined, then execute someFunction with this.b and this.c
if (!this.a) {
    someFunction(this.b!, this.c!)    
}

// alternatively, only call someFunction if both this.b and this.c are assigned strings
if(this.b && this.c){
    someFunction(this.b, this.c)
}

Link to Interactive Playground

Writing for: Either attribute "a" is defined or attributes "b" and "c" are both defined

Answer №1

Answer №2

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