How to Gracefully Pause Execution Until an Asynchronous Function Completes?

Proper Way to Pause Execution Until a Function Completes

When working with multiple asynchronous functions, it's often necessary to wait for one function to finish before executing another. Let's examine a solution and explore more elegant approaches.

Original Solution

The provided solution uses a polling mechanism with setTimeout to continuously check if the first function has completed:

<code class="javascript">var isPaused = false;

function firstFunction() {
    isPaused = true;
    // Do something
    isPaused = false;
}

function secondFunction() {
    firstFunction();
    function waitForIt() {
        if (isPaused) {
            setTimeout(waitForIt, 100);
        } else {
            // Do something else
        }
    }
}</code>

While this method can work, it's not optimal due to its reliance on polling.

Elegant Approaches

Callback Functions:

A common practice is to use callback functions to handle asynchronous completion:

<code class="javascript">function firstFunction(callback) {
    // Do asynchronous work
    callback();
}

function secondFunction() {
    firstFunction(() => {
        console.log("huzzah, I'm done!");
    });
}</code>

When firstFunction finishes, it calls the callback function, allowing secondFunction to continue executing.

Arrow Functions:

Using arrow functions simplifies this approach:

<code class="javascript">firstFunction(() => console.log('huzzah, I'm done!'))</code>

Async/Await:

In modern JavaScript, async/await offers a more readable and efficient way to pause execution:

<code class="javascript">const secondFunction = async () => {
  const result = await firstFunction()
  // Do something else here after firstFunction completes
}</code>

Conclusion

While polling can be used to wait for function completion, callback functions, arrow functions, and async/await provide more elegant and performant solutions. Choose the approach that best suits your specific requirements and JavaScript environment.

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