calling async function from non async

Daniel Parker

3 min read

·

26-02-2025

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Asynchronous functions (async functions) are a cornerstone of modern JavaScript, enabling efficient handling of operations that might take time to complete, like network requests or file I/O. But what happens when you need to call an async function from a regular (non-async) function? This is a common scenario, and understanding how to do it correctly is crucial for writing robust and efficient JavaScript code. This article will explain the methods and best practices for calling async functions from within synchronous contexts.

Understanding the Problem: Async/Await and Promises

async functions implicitly return a Promise. If you simply call an async function from a sync function, you won't get the result directly. Instead, you'll get a Promise object representing the eventual result of the asynchronous operation. This is where the await keyword (which can only be used inside async functions) comes in. await pauses execution until the Promise resolves (or rejects).

Let's illustrate this with an example:

async function myAsyncFunction() {
  return await new Promise(resolve => setTimeout(() => resolve("Hello from async!"), 1000));
}

function mySyncFunction() {
  const result = myAsyncFunction(); // result is a Promise, not "Hello from async!"
  console.log(result); // Outputs: Promise { <pending> }
  console.log("Sync function continues immediately."); 
}

mySyncFunction(); 

In this example, mySyncFunction calls myAsyncFunction, but the console.log immediately after shows that the Promise is still pending. The synchronous function continues execution without waiting for the async function to finish.

Methods for Handling Async Calls in Sync Functions

There are two primary approaches to handle this:

1. .then() and .catch() (Promise chaining)

The most straightforward way to manage asynchronous operations within a synchronous function is to use the .then() method to handle the resolved Promise and the .catch() method to handle potential errors.

async function myAsyncFunction() {
  return await new Promise(resolve => setTimeout(() => resolve("Hello from async!"), 1000));
}

function mySyncFunction() {
  myAsyncFunction()
    .then(result => {
      console.log("Async function completed:", result); // Outputs: Async function completed: Hello from async! after 1 second
    })
    .catch(error => {
      console.error("Error in async function:", error);
    });
  console.log("Sync function continues immediately."); // Still runs first
}

mySyncFunction();

This method keeps the mySyncFunction synchronous while still handling the result of the asynchronous operation. The console.log within the .then() block will only execute after myAsyncFunction resolves.

2. Async/Await with a Wrapper

Another approach is to wrap the call to the asynchronous function within another async function. This allows you to use await to pause execution until the async operation is complete before returning the result.

async function myAsyncFunction() {
    return await new Promise(resolve => setTimeout(() => resolve("Hello from async!"), 1000));
  }
  
  async function myAsyncWrapper() {
    const result = await myAsyncFunction();
    return result;
  }
  
  function mySyncFunction() {
    myAsyncWrapper()
    .then(result => {
        console.log("Result from async wrapper:", result);
    })
    .catch(error => {
        console.error("Error in async wrapper:", error);
    });
    console.log("Sync function continues immediately.");
  }
  
  mySyncFunction();

This approach provides a cleaner way to handle the asynchronous operation, especially when dealing with multiple asynchronous calls. You can await multiple promises sequentially within your async wrapper function.

Best Practices

• Error Handling: Always include .catch() blocks to handle potential errors during asynchronous operations.
• Keep it Clean: For simple scenarios, .then()/.catch() might suffice. For more complex logic involving multiple async calls, wrapping in another async function and using await can lead to clearer, more readable code.
• Context: Remember that the this keyword will behave differently inside an async function compared to a regular function. Be mindful of the context in which your functions are called.

By understanding these methods and adhering to best practices, you can effectively manage and integrate async functions into your synchronous JavaScript code. Choosing the best approach depends on the complexity of your application logic and personal preference, but both methods ultimately achieve the same result: executing asynchronous code safely and handling its results within a synchronous context.