Tips on accessing fields for union types that are passed to a function in TypeScript

Question

Tips on accessing fields for union types that are passed to a function in TypeScript

I'm currently diving into the world of TypeScript, teaching myself by putting my newfound knowledge into practice.

Below is the code I'm working on, but I've hit a roadblock trying to access the fields of the type passed to the function.

Code :


interface Product1 {
    name: string;
    category: string;
}

interface Product2 {
    name: string;
    price: number;
}


type Product = Product1 | Product2;

function DisplayProductDetails(data: Product) {
    const keys : string[] = Object.keys(data);
    keys.forEach((key) => {
        console.log(data[key])
    })
}

const p2 = {
    name: "iPhone",
    price: 999
}

DisplayProductDetails(p2)

Error: The error is on this line - console.log(data[key])

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Product'. No index signature with a parameter of type 'string' was found on type 'Product'.

typescript

Answer 1

Answer №1

When attempting to utilize a string as a key for the OverView type, you may encounter issues since this type only supports 3 keys: name, family, storage. The compiler may raise concerns about the index access due to the versatile nature of the string. Even if you eliminate the explicit string[] from the keys type, the error persists because Object.keys does not provide generic support and consistently yields an array of string.

To address this, consider the following solutions:

Construct a typed version of Object.keys
Utilize as assertion to convert the keys' type to an array of keys from OverView

For this response, the second solution will be implemented:

function LogKeyValuePair(data: OverView) {
  const keys = Object.keys(data) as (keyof OverView)[];
  keys.forEach((key) => {
    console.log(data[key]); // no error
  });
}

It is important to note that, given the union nature of OverView, the resulting keys array will solely consist of name as it represents the common key within the unions.

Answer 2

When attempting to utilize a string as a key for the OverView type, you may encounter issues since this type only supports 3 keys: name, family, storage. The compiler may raise concerns about the index access due to the versatile nature of the string. Even if you eliminate the explicit string[] from the keys type, the error persists because Object.keys does not provide generic support and consistently yields an array of string.

To address this, consider the following solutions:

Construct a typed version of Object.keys
Utilize as assertion to convert the keys' type to an array of keys from OverView

For this response, the second solution will be implemented:

function LogKeyValuePair(data: OverView) {
  const keys = Object.keys(data) as (keyof OverView)[];
  keys.forEach((key) => {
    console.log(data[key]); // no error
  });
}

It is important to note that, given the union nature of OverView, the resulting keys array will solely consist of name as it represents the common key within the unions.

Answer 3

Answer №2

One issue is that the key type (which is a string) lacks specificity when it comes to indexing data (in this case, OverView). To properly index the data, you would require keyof OverView. However, Object.keys() only gives back an array of generic string values. One potential solution would be to utilize a type assertion in this scenario.

interface Overview1 {
  name: string;
  family: string;
}

interface Overview2 {
  name: string;
  storage: string;
}

type OverView = Overview1 | Overview2;

function LogKeyValuePair(data: OverView) {
  const keys = Object.keys(data) as (keyof OverView)[];
  keys.forEach((key) => {
    console.log(data[key]);
  });
}

const o2 = {
  name: "john",
  storage: "10 gb",
};

LogKeyValuePair(o2);

TypeScript Playground

Answer 4

One issue is that the key type (which is a string) lacks specificity when it comes to indexing data (in this case, OverView). To properly index the data, you would require keyof OverView. However, Object.keys() only gives back an array of generic string values. One potential solution would be to utilize a type assertion in this scenario.

interface Overview1 {
  name: string;
  family: string;
}

interface Overview2 {
  name: string;
  storage: string;
}

type OverView = Overview1 | Overview2;

function LogKeyValuePair(data: OverView) {
  const keys = Object.keys(data) as (keyof OverView)[];
  keys.forEach((key) => {
    console.log(data[key]);
  });
}

const o2 = {
  name: "john",
  storage: "10 gb",
};

LogKeyValuePair(o2);

TypeScript Playground

Answer 5

Answer №3

An issue has occurred: the error is on this line console.log(data[key]) Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'OverView'. No index signature with a parameter of type 'string' was found on type 'OverView'.

Due to the fact that keys are defined as type string[], TypeScript is unable to determine if the key belongs to the Overview type. Here are some potential solutions to resolve this error.

Specify the type using the as keyword

By including this code

console.log(data[key as keyof OverView])

, TypeScript will recognize that the key is within the Overview type.

Note! While this solution may work, it can result in runtime errors. It is advisable to verify null values before accessing properties in the data object.

interface Overview1 {
    name: string;
    family: string;
}

interface Overview2 {
    name: string;
    storage: string;
}


type OverView = Overview1 | Overview2;

function LogKeyValuePair(data: OverView) {
    const keys : string[] = Object.keys(data);
    keys.forEach((key) => {
      console.log(data[key as keyof OverView])
    })
}

const o2 = {
    name: "john",
    storage: "10 gb"
}

LogKeyValuePair(o2)

Alternatively, specify the type when assigning the array of keys

function LogKeyValuePair(data: OverView) {
    const keys = Object.keys(data) as (keyof OverView)[];
    keys.forEach((key) => {
      console.log(data[key])
    })
}

Playground link

Answer 6

An issue has occurred: the error is on this line console.log(data[key]) Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'OverView'. No index signature with a parameter of type 'string' was found on type 'OverView'.

Due to the fact that keys are defined as type string[], TypeScript is unable to determine if the key belongs to the Overview type. Here are some potential solutions to resolve this error.

Specify the type using the as keyword

By including this code

console.log(data[key as keyof OverView])

, TypeScript will recognize that the key is within the Overview type.

Note! While this solution may work, it can result in runtime errors. It is advisable to verify null values before accessing properties in the data object.

interface Overview1 {
    name: string;
    family: string;
}

interface Overview2 {
    name: string;
    storage: string;
}


type OverView = Overview1 | Overview2;

function LogKeyValuePair(data: OverView) {
    const keys : string[] = Object.keys(data);
    keys.forEach((key) => {
      console.log(data[key as keyof OverView])
    })
}

const o2 = {
    name: "john",
    storage: "10 gb"
}

LogKeyValuePair(o2)

Alternatively, specify the type when assigning the array of keys

function LogKeyValuePair(data: OverView) {
    const keys = Object.keys(data) as (keyof OverView)[];
    keys.forEach((key) => {
      console.log(data[key])
    })
}

Playground link

Tips on accessing fields for union types that are passed to a function in TypeScript

Answer №1

Answer №2

Answer №3

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