In Typescript, every expression inherently holds a value

Question

In Typescript, every expression inherently holds a value

I have a specific interface set up as shown below:

interface Actions {
  run: any;
  execute: any;
}

const actions: Actions = {
  run: require("./runAction"),
  execute: require("./executeAction"),
}

My goal is to call the functions associated with run and execute using the code snippet below:

const result = await actions[action_type]();

Here, action_type represents a string containing the name of the action (run/execute). However, I am encountering the following error message:

TS7053: Element implicitly has an 'any' type because expression of type 'any' can't be used to index type 'Actions'

Any suggestions on how to resolve this issue?

typescript

Answer 1

Answer №1

There are a couple of distinct issues:

The declarations for copy and move in TypeScript specify the type as any, but you actually want to treat them as functions.
You intend to utilize an arbitrary string (command_type) to access elements in commands. However, due to the nature of arbitrary strings potentially being anything (not just restricted to "copy" or "move"), directly using an arbitrary string in a typesafe manner with your current interface presents a challenge.

To address the typing problem, defining a function type instead of any can clarify to TypeScript that these are indeed functions. For example, you could define it as

() => Promise<Something>

for a function that doesn't take arguments and returns a promise.

In terms of indexing, you have two potential solutions:

Rather than allowing command_type to be any string, align it with the types of properties in command: keyof Commands. This might involve using atype predicate function, a type assertion function, or both. Here's how they can be implemented:

// Type predicate function: validates if `type` is a valid key
function isCommandType(type: string): type is keyof Commands {
     return type === "copy" || type === "move";
}

// Type assertion function: throws an error for invalid `type`
function assertIsCommandType(type: string): asserts type is keyof Commands {
     if (!isCommandType(type)) {
         throw new Error(`Expected "copy" or "move", got ${JSON.stringify(type)}`);
     }
}

By starting with any string (e.g., req.body.action), you can employ the type predicate function like this:

const command_type = req.body.action;
if (isCommandType(command_type)) {
     const result = await commands[command_type]();
     // ...
}

...or you could use the type assertion function to enforce correctness:

const command_type = req.body.action;
assertIsCommandType(command_type);
const result = await commands[command_type]();
// ...

Alternatively, if you prefer command_type to accept any string, your interface can be altered as follows:
```
interface Commands {
     [key: string]: () => Promise<Something>;
}
```
This adjustment results in commands[command_type] having a type of (() => void)|undefined (with undefined accounting for cases where the string does not match any property). The call would look like this:
```
const result = await commands[command_type]?.();
```
As a consequence, result becomes undefined | Something due to optional chaining returning undefined when commands[command_type] is either undefined or null.

You can also separate access from invocation, indicating to TypeScript that the handling of undefined has been managed:
```
const command = commands[command_type];
if (command) {
     const result = await command();
     // ...
}
```
In this scenario, result assumes the type Something based on the return type Promise<Something>.

Answer 2

There are a couple of distinct issues:

The declarations for copy and move in TypeScript specify the type as any, but you actually want to treat them as functions.
You intend to utilize an arbitrary string (command_type) to access elements in commands. However, due to the nature of arbitrary strings potentially being anything (not just restricted to "copy" or "move"), directly using an arbitrary string in a typesafe manner with your current interface presents a challenge.

To address the typing problem, defining a function type instead of any can clarify to TypeScript that these are indeed functions. For example, you could define it as

() => Promise<Something>

for a function that doesn't take arguments and returns a promise.

In terms of indexing, you have two potential solutions:

Rather than allowing command_type to be any string, align it with the types of properties in command: keyof Commands. This might involve using atype predicate function, a type assertion function, or both. Here's how they can be implemented:

// Type predicate function: validates if `type` is a valid key
function isCommandType(type: string): type is keyof Commands {
     return type === "copy" || type === "move";
}

// Type assertion function: throws an error for invalid `type`
function assertIsCommandType(type: string): asserts type is keyof Commands {
     if (!isCommandType(type)) {
         throw new Error(`Expected "copy" or "move", got ${JSON.stringify(type)}`);
     }
}

By starting with any string (e.g., req.body.action), you can employ the type predicate function like this:

const command_type = req.body.action;
if (isCommandType(command_type)) {
     const result = await commands[command_type]();
     // ...
}

...or you could use the type assertion function to enforce correctness:

const command_type = req.body.action;
assertIsCommandType(command_type);
const result = await commands[command_type]();
// ...

Alternatively, if you prefer command_type to accept any string, your interface can be altered as follows:
```
interface Commands {
     [key: string]: () => Promise<Something>;
}
```
This adjustment results in commands[command_type] having a type of (() => void)|undefined (with undefined accounting for cases where the string does not match any property). The call would look like this:
```
const result = await commands[command_type]?.();
```
As a consequence, result becomes undefined | Something due to optional chaining returning undefined when commands[command_type] is either undefined or null.

You can also separate access from invocation, indicating to TypeScript that the handling of undefined has been managed:
```
const command = commands[command_type];
if (command) {
     const result = await command();
     // ...
}
```
In this scenario, result assumes the type Something based on the return type Promise<Something>.

In Typescript, every expression inherently holds a value

Answer №1

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