TypeScript Promises

When working with TypeScript, understanding promises is key to handling asynchronous tasks effectively. Promises offer a clear way to manage asynchronous code, making it easier to read and maintain. By mastering promises, you can write more reliable and clean code. In this guide, we'll explore how to use promises in TypeScript, covering creation, chaining, error handling, and more.

Creating a Promise in TypeScript

To create a promise in TypeScript, use the Promise constructor. This constructor takes a function with resolve and reject parameters, allowing you to define the asynchronous operation within the promise.

const promise = new Promise((resolve, reject) => {
  // Asynchronous operation
  setTimeout(() => {
    resolve("Promise resolved");
  }, 2000);
});

promise.then((value) => {
  console.log(value); // Outputs: Promise resolved
});

Modern TypeScript applications, like those using Convex's BaseConvexClient, rely heavily on properly typed promises for handling asynchronous operations.

Handling Asynchronous Operations with Promises

Promises can be chained using the then() method to manage sequential asynchronous tasks. This approach is useful when you need to fetch data, process it, and display the result. Building robust asynchronous workflows often requires careful error handling with TypeScript try catch alongside promise chains.

const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("First promise resolved");
  }, 2000);
});

promise
  .then((value) => {
    console.log(value); // Outputs: First promise resolved
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve("Second promise resolved");
      }, 2000);
    });
  })
  .then((value) => {
    console.log(value); // Outputs: Second promise resolved
  });

When building applications with frameworks like Convex, you can use promise chains to manage data flow as demonstrated in their Functional Relationships Helpers guide.

Using Async/Await with Promises

The async/await syntax simplifies working with promises by letting you write asynchronous code that looks synchronous. This makes your code easier to read and maintain.

async function asyncOperation() {
  const promise = new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("Promise resolved");
    }, 2000);
  });

  const value = await promise;
  console.log(value); // Outputs: Promise resolved
}

asyncOperation();

When using async/await, proper error handling with TypeScript try catch becomes essential for managing rejected promises. Async functions always return a promise, making them seamlessly compatible with Convex's asynchronous data patterns described in their Code Spelunking API Guide.

Chaining Multiple Promises

Linking multiple promises lets you handle complex asynchronous workflows. Each promise in the chain can do a specific task, with results passed to the next promise.

const promise1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("Promise 1 resolved");
  }, 2000);
});

const promise2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("Promise 2 resolved");
  }, 2000);
});

promise1
  .then((value) => {
    console.log(value); // Outputs: Promise 1 resolved
    return promise2;
  })
  .then((value) => {
    console.log(value); // Outputs: Promise 2 resolved
  });

When managing complex data relationships in Convex applications, promise chains are essential as shown in their Functional Relationships Helpers guide. For more advanced type safety in promise chains, consider using TypeScript generics to specify return types at each step.

Handling Errors in Promises

Proper error handling in promises is essential to prevent application crashes from unhandled rejections. The catch() method is used to catch and manage any errors that occur in the promise chain.

const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    reject("Promise rejected");
  }, 2000);
});

promise
  .then((value) => {
    console.log(value);
  })
  .catch((error) => {
    console.log(error); // Outputs: Promise rejected
  });

Effective error handling in TypeScript promises requires understanding the TypeScript try catch pattern. The catch() method in promise chains serves a similar purpose to try/catch blocks in synchronous code.

Typing Promises for Specific Return Values

TypeScript allows you to define the type of data a promise will resolve to, improving type safety and code completion.

interface User {
  id: number;
  name: string;
}

const promise: Promise<User> = new Promise((resolve, reject) => {
  // Asynchronous operation to fetch user data
  setTimeout(() => {
    resolve({ id: 1, name: "John Doe" });
  }, 2000);
});

promise.then((user: User) => {
  console.log(user.id); // Outputs: 1
  console.log(user.name); // Outputs: John Doe
});

When defining promise types, you're essentially using TypeScript generics with the Promise type. The TypeScript interface example above demonstrates how to structure complex data types for promises.

Converting Callback Functions to Promises

Turning callback functions into promises can make your code more organized and easier to read, especially when dealing with asynchronous operations.

function callbackFunction(callback: (error: any, data: any) => void) {
  // Asynchronous operation
  setTimeout(() => {
    callback(null, "Data from callback function");
  }, 2000);
}

const promise = new Promise((resolve, reject) => {
  callbackFunction((error, data) => {
    if (error) {
      reject(error);
    } else {
      resolve(data);
    }
  });
});

promise.then((value) => {
  console.log(value); // Outputs: Data from callback function
});

This conversion pattern transforms legacy callback-based code into modern promise-based syntax. When working with function parameters in TypeScript, understanding TypeScript types helps ensure proper type definitions. The transformation process becomes more powerful when combined with TypeScript utility types to create flexible, type-safe conversions.

Final Thoughts about TypeScript Promises

Learning how to create, chain, and handle errors in promises dramatically improves your development experience. Used correctly, promises offer a robust tool for managing asynchronous complexity. With practice, promises become an essential part of your toolkit for building scalable applications.