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Topic 021: Node.js: A Step-by-Step Guide to Handling Asynchronous JWT Verification

Topic 021: Node.js: A Step-by-Step Guide to Handling Asynchronous JWT Verification

Introduction

When working with asynchronous functions in JavaScript, especially in Node.js, handling callbacks can become challenging. A common scenario is using jwt.verify to decode tokens. Without properly handling asynchronous behavior, you might encounter issues like undefined being returned, even though you're expecting a valid decoded token.

In this guide, we will explore how to handle such cases using async/await with jwt.verify, ensuring that your code runs in sequence and provides the expected results.

Common Issue: await Not Working with jwt.verify

In an asynchronous function, you might try using await on a function like jwt.verify, expecting the token to be decoded and returned. However, jwt.verify uses a callback-based API. This means that without properly handling the callback, the return value could be undefined, leading to unexpected behavior.

Here’s an example of what can go wrong:

export const onLogin = async (event, api) => {
  const userId = await validateToken(api, event); // Attempt to await result
  console.log("User ID:", userId); // Unexpectedly logs `undefined`
};

In this case, the function validateToken is not returning the decoded token correctly because jwt.verify's callback isn't being awaited properly. Let's see how we can fix this.

Solution: Wrapping jwt.verify in a Promise

Since jwt.verify uses a callback, you can wrap it inside a Promise to make it compatible with async/await. This allows you to handle the result (or error) in a sequential manner.

Example:

Here’s how you can refactor your validateToken function to work with async/await:

const jwt = require("jsonwebtoken"); // Assuming you're using jsonwebtoken

async function validateToken(api, event) {
  const idToken = api.idToken;
  const getKey = api.getKey;
  const audience = "your_audience";
  const issuer = "your_issuer";
  const algorithms = ["RS256"];

  return new Promise((resolve, reject) => {
    jwt.verify(idToken, getKey, { audience, issuer, algorithms }, (err, decoded) => {
      if (err) {
        return reject(err); // Handle errors
      }
      resolve(decoded); // Return the decoded token
    });
  });
}

Explanation:

  1. Promise Wrapper: We wrap jwt.verify inside a Promise. If the verification is successful, the promise resolves with the decoded token. Otherwise, it rejects with an error.
  2. async/await: By returning a Promise, you can now use await to pause execution until the promise resolves, ensuring sequential execution.

Usage Example in onLogin Function

Now that validateToken returns a Promise, you can use await in your onLogin function to ensure the token is decoded before moving on.

export const onLogin = async (event, api) => {
  try {
    const decodedToken = await validateToken(api, event); // Wait for token verification
    console.log("Decoded Token:", decodedToken); // Successfully prints decoded token
  } catch (error) {
    console.error("Error verifying token:", error); // Handle any errors
  }
};

What This Does:

  • Sequential Execution: The await ensures that the token is decoded before moving to the next line.
  • Error Handling: Any errors during the token verification process are caught and handled properly.
  • Clean, Readable Code: Using async/await improves code readability and reduces callback complexity.

Further Reading: