Navigating the world of Typescript: mastering union types and handling diverse attributes

Question

Navigating the world of Typescript: mastering union types and handling diverse attributes

I am currently working on building a function that can accept two different types of input.

type InputA = {
 name: string
 content: string
 color: string
}

type InputB = {
 name: string
 content: number
}

type Input = InputA | InputB

As I try to implement this function to handle both types of inputs, my goal is to differentiate between the two based on whether they have the color attribute present.

However, I keep encountering a compiler error:


function foo(input:Input){
 const color = (input.color) ? input.color : undefined;
                  // ^
                  // | Property 'color' does not exist on type 'Input'.
                  // | Property 'color' does not exist on type 'InputB'.
 ...
}

I am aware that one solution would be to add a new attribute, such as type_name, that exists in both types and then check against that. However, I prefer to avoid adding extra attributes solely for this purpose within this specific function.

Is there a more efficient way to approach this issue?

typescript types union-types

Answer 1

Answer №1

To ensure type safety, it is not possible to access properties that may not exist on a union type using the dot notation. Instead, use the in operator.

function foo(input:Input){
  const color = "color" in input ? input.color : undefined
}

Playground

Answer 2

To ensure type safety, it is not possible to access properties that may not exist on a union type using the dot notation. Instead, use the in operator.

function foo(input:Input){
  const color = "color" in input ? input.color : undefined
}

Playground

Answer 3

Answer №2

There is a potential for exploring alternative methods of utilizing type predicates. By defining the type identification logic, these predicates can be used for purposes beyond just identifying color properties.

// Determines if the variable is of type InputA
const isOfTypeInputA = (input: Input): input is InputA => {
    return input.hasOwnProperty("color");
}

function foo(input:Input){
 const color = isOfTypeInputA(input) ? input.color : undefined;
}

For a functional solution, visit: Typescritp playground link

Answer 4

There is a potential for exploring alternative methods of utilizing type predicates. By defining the type identification logic, these predicates can be used for purposes beyond just identifying color properties.

// Determines if the variable is of type InputA
const isOfTypeInputA = (input: Input): input is InputA => {
    return input.hasOwnProperty("color");
}

function foo(input:Input){
 const color = isOfTypeInputA(input) ? input.color : undefined;
}

For a functional solution, visit: Typescritp playground link

Navigating the world of Typescript: mastering union types and handling diverse attributes

Answer №1

Answer №2

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