Promise returning undefined in Node.js | async function Node.js

Node.js / September 9, 2021 / TopCode / 0

Introduction

How to resolve a promise in Node.js? This is a very basic programming technique for a JavaScript developer in Node.js. In this blog post, you will learn best practices to use the result of asynchronous functions. What is the impact of not using await in Node.js.

In Node.js, async function always returns a promise object. To obtain the underlying value of the promise, you should use the await operator or call the then method. Attempting to use a promise object instead of its underlying value can lead to an unexpected result.

Example in Node.js

In the following example, the getResult the function returns a promise, and in the caller function there is a check if the returned promise is undefined:

async function getResult(id) {
  let req = await fetch(`https://topcode.in/data?id=${id}`);
  if (!req.ok) return null;
  return req.json();
}

async function foo(id) {
  let data = getResult(id);       //storing result of async function without await or then() 
  if (!data) {
    console.log("No data for: " + id);
    return;
  }
}

Recommendation

Use the await operator to get the value contained in the promise. Alternatively, you can call then method on the promise and use the value passed to the callback.

Following is the solution of the problem:

async function getResult(id) {
  let req = await fetch(`https://topcode.in/data?id=${id}`);
  if (!req.ok) return null;
  return req.json();
}

async function foo(id) {
  let data = await getResult(id);       //here using await keyword 
  if (!data) {
    console.log("No data for: " + id);
    return;
  }
}

References

• MDN: Using promises
• MDN: Async functions
• MDN: Await operator

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