Steer clear of using enum values in typescript to prevent potential issues

Question

Steer clear of using enum values in typescript to prevent potential issues

I am looking to iterate through an enum type in order to populate options within a react component. Below, you will find the specific enum and a function that retrieves its keys and values.

export enum TaskType {
  daily, 
  weekly, 
  monthly, 
  yearly
}

function taskTypeValues() {
  const values = Object.keys(TaskType).filter(k => typeof TaskType[k as any] === "number"); // ["daily", "weekly", "monthly", "yearly"]
  const keys = values.map(k => TaskType[k as any]); // [0, 1, 2, 3]
}

I’m not comfortable using the any type in TaskType[k as any], so I attempted:

type EnumIndex = number | string;
TaskType[k as EnumIndex]

However, I received this error:

Element implicitly has an 'any' type because index expression is not of type 'number'.ts(7015)

I believed that the indexer could be either a number or string, since Object.Keys returns an array with 8 elements:

["0","1","2","3","daily","weekly","monthly","yearly"]

, but neither approach worked.

How can I eliminate the any type in this scenario when the enum types are already defined?

typescript

Answer 1

Answer №1

This particular issue stems from the fact that the type signature for Object.keys(obj) specifies the return type as string[] rather than something like Array<keyof typeof obj>.

The reason behind this design choice is that object types in TypeScript are considered to be open instead of closed; they may have a set of known properties, but there is also room for the possibility of having additional properties. For further insights on this topic, you can refer to the Q/A discussion titled "Why doesn't Object.keys return a keyof type in TypeScript?".

Hence, when attempting to index into the ProyectType with an arbitrary string, the compiler raises concerns about it. In scenarios where the existence of additional keys is not a concern or if you possess knowledge about the specific keys, employing a type assertion allows you to explicitly inform the compiler about what cannot be inferred.

For instance, consider this approach:

type ProyectTypeKeys = Array<keyof typeof ProyectType | number>;
// type ProyectTypeKeys = (number | "time" | "hits" | "value" | "results")[]

const values = (Object.keys(ProyectType) as ProyectTypeKeys).
    filter((k): k is keyof typeof ProyectType =>
        typeof ProyectType[k] === "number");
// const values: ("time" | "hits" | "value" | "results")[]

const keys = values.map(k => ProyectType[k]);
// const keys: ProyectType[]

In this code snippet, I define ProyectTypeKeys as an array type comprising either the established string keys of the enum object or number... essentially covering your expected scenario and explaining the purpose behind the filter() operation.

Similarly, I specify the filter() callback as a user-defined type guard function to guide the compiler in using a call signature for filter() that refines the output type from the comprehensive array type ProyectTypeKeys to solely the desired type: keyof typeof ProyectType, which equates to

"time" | "hits" | "value" | "results"

.

Following this sequence, the derived type of keys aligns with ProyectType[], meeting the required specifications.

Access Playground link for source code

Answer 2

This particular issue stems from the fact that the type signature for Object.keys(obj) specifies the return type as string[] rather than something like Array<keyof typeof obj>.

The reason behind this design choice is that object types in TypeScript are considered to be open instead of closed; they may have a set of known properties, but there is also room for the possibility of having additional properties. For further insights on this topic, you can refer to the Q/A discussion titled "Why doesn't Object.keys return a keyof type in TypeScript?".

Hence, when attempting to index into the ProyectType with an arbitrary string, the compiler raises concerns about it. In scenarios where the existence of additional keys is not a concern or if you possess knowledge about the specific keys, employing a type assertion allows you to explicitly inform the compiler about what cannot be inferred.

For instance, consider this approach:

type ProyectTypeKeys = Array<keyof typeof ProyectType | number>;
// type ProyectTypeKeys = (number | "time" | "hits" | "value" | "results")[]

const values = (Object.keys(ProyectType) as ProyectTypeKeys).
    filter((k): k is keyof typeof ProyectType =>
        typeof ProyectType[k] === "number");
// const values: ("time" | "hits" | "value" | "results")[]

const keys = values.map(k => ProyectType[k]);
// const keys: ProyectType[]

In this code snippet, I define ProyectTypeKeys as an array type comprising either the established string keys of the enum object or number... essentially covering your expected scenario and explaining the purpose behind the filter() operation.

Similarly, I specify the filter() callback as a user-defined type guard function to guide the compiler in using a call signature for filter() that refines the output type from the comprehensive array type ProyectTypeKeys to solely the desired type: keyof typeof ProyectType, which equates to

"time" | "hits" | "value" | "results"

.

Following this sequence, the derived type of keys aligns with ProyectType[], meeting the required specifications.

Access Playground link for source code

Answer 3

Answer №2

It's quite peculiar how this issue seems to only arise in certain functions.

Consider taking a closer look at the following implementation:

const keys = [];
const values = [];
for(const index in ProyectType){
   //typescript infers string to index const  but here there is no error
  typeof ProyectType[index] === "number" ? values.push(index) : keys.push(index)
 
}

To maintain the current structure of your code without encountering any errors, one potential solution involves using the unknown type along with number.

const values = Object.keys(ProyectType).filter(k => typeof ProyectType[k as unknown as number] === "number"); // ["time", "hits", "value", "results"]

Answer 4

It's quite peculiar how this issue seems to only arise in certain functions.

Consider taking a closer look at the following implementation:

const keys = [];
const values = [];
for(const index in ProyectType){
   //typescript infers string to index const  but here there is no error
  typeof ProyectType[index] === "number" ? values.push(index) : keys.push(index)
 
}

To maintain the current structure of your code without encountering any errors, one potential solution involves using the unknown type along with number.

const values = Object.keys(ProyectType).filter(k => typeof ProyectType[k as unknown as number] === "number"); // ["time", "hits", "value", "results"]

Answer 5

Answer №3

Utilizing Type Assertions for Efficient Coding

Here's a clever and innovative approach :) Given that enums are limited to only string and number types, and assuming your intention is clear, type assertions can be employed to retrieve values and keys with finesse. Type casting (or assertion) serves as a signal to the compiler that you're in control. For more detailed explanations, refer to this resource.

In this particular scenario, your function ought to look something like this:

function extractTypeData() {
  const values = Object.values(ProyectType)
                .filter((val) => isNaN(val as number));
  const keys = Object.keys(ProyectType)
              .filter(key => !isNaN(key as unknown as number))
              .map(key => +key);
}

The usage of key as unknown as number is necessary here since our keys are in the form of strings. The mapping process is then applied to convert these strings into numerical values as per your requirement.

Answer 6

Utilizing Type Assertions for Efficient Coding

Here's a clever and innovative approach :) Given that enums are limited to only string and number types, and assuming your intention is clear, type assertions can be employed to retrieve values and keys with finesse. Type casting (or assertion) serves as a signal to the compiler that you're in control. For more detailed explanations, refer to this resource.

In this particular scenario, your function ought to look something like this:

function extractTypeData() {
  const values = Object.values(ProyectType)
                .filter((val) => isNaN(val as number));
  const keys = Object.keys(ProyectType)
              .filter(key => !isNaN(key as unknown as number))
              .map(key => +key);
}

The usage of key as unknown as number is necessary here since our keys are in the form of strings. The mapping process is then applied to convert these strings into numerical values as per your requirement.

Steer clear of using enum values in typescript to prevent potential issues

Answer №1

Answer №2

Answer №3

Utilizing Type Assertions for Efficient Coding

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