How Do I Access Previous Promise Results in A .Then() Chain?

When working with JavaScript promises, it is common to have a chain of .then() functions to handle asynchronous operations. However, there may be cases where you need to access the results of previous promises within the chain. In this blog post, we will explore different approaches to accessing previous promise results in a .then() chain.

Approach 1: Using Variables

One straightforward way to access previous promise results is by storing them in variables within the .then() chain. This allows you to reference the previous results in subsequent .then() functions. Here’s an example:

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => {
    // Store the result in a variable
    const result1 = data;
    return fetch('https://api.example.com/another-data');
  })
  .then(response => response.json())
  .then(data => {
    // Access the previous result using the variable
    const result2 = data;
    console.log(result1, result2);
  });

In this example, we store the result of the first fetch request in the result1 variable. Then, we make another fetch request and store its result in the result2 variable. Finally, we log both results to the console.

Approach 2: Chaining Promises

Another approach is to chain the promises together using the .then() method. This allows you to pass the previous result as an argument to the next .then() function. Here’s an example:

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => {
    return fetch('https://api.example.com/another-data')
      .then(response => response.json())
      .then(anotherData => {
        // Access the previous result using the argument
        console.log(data, anotherData);
      });
  });

In this example, we chain the second fetch request directly within the first .then() function. The result of the first fetch request (data) is passed as an argument to the second .then() function, allowing us to access it along with the result of the second fetch request (anotherData).

Approach 3: Using async/await

If you prefer a more synchronous-looking code, you can use async/await syntax to access previous promise results. Here’s an example:

async function fetchData() {
  const response1 = await fetch('https://api.example.com/data');
  const data1 = await response1.json();

  const response2 = await fetch('https://api.example.com/another-data');
  const data2 = await response2.json();

  console.log(data1, data2);
}

fetchData();

In this example, we define an async function fetchData() and use the await keyword to wait for each promise to resolve before moving on to the next line of code. This allows us to access the results of each fetch request directly using variables (data1 and data2).

These are three different approaches to accessing previous promise results in a .then() chain. Choose the one that best suits your coding style and requirements. Happy coding!