The reliability of innerHTML cannot be guaranteed as it does not consistently run synchronously

Question

The reliability of innerHTML cannot be guaranteed as it does not consistently run synchronously

For a live demonstration of the issue, visit this jsbin. When the button is clicked, it triggers the buttonHandler() function, which is outlined below:

function buttonHandler() {
  var elm = document.getElementById("progress");
  elm.innerHTML = "thinking";
  longPrimeCalc();
}

The expectation is that this code should update the div text to "thinking" and then initiate the execution of longPrimeCalc(), a complex arithmetic operation that takes a few seconds to be processed. Surprisingly, the actual behavior observed is different. The "longPrimeCalc" operation completes first before updating the text to "thinking," almost as if the sequence of these two lines in the code has been reversed.

It seems like the browser doesn't handle the "innerHTML" code synchronously; instead, it creates a separate thread for its execution at its own pace.

So, here are my inquiries:

What exactly is going on behind the scenes that causes this unexpected behavior?
Is there a way to compel the browser to behave as intended and update the "innerHTML" before proceeding with "longPrimeCalc()"?

This test was conducted using the latest version of Google Chrome.

javascript

Answer 1

Answer №1

Your assumption is incorrect. The update of .innerHTML actually occurs synchronously (and there is no new thread created by the browser). Instead, the browser simply delays updating the window until your code has finished running. If you were to inspect the DOM in a way that necessitated an update to the view, then the browser would have to do so.

For instance, right after setting the innerHTML, include this line:

var height = element.clientHeight; // Oops, wrong... let me check again.

edit — I may discover a workaround to deceive the browser, or it could be impossible. It's definitely accurate that running your lengthy computation in a separate event loop will make it function:

setTimeout(longCalculation, 10); // Avoid using 0, especially with Firefox!

An important lesson here is that browsers strive to avoid unnecessary reflows of the page layout. If your code had taken a detour to find prime numbers and then returned to update the innerHTML once more, the browser would have saved itself from performing unnecessary work. Even if it doesn't visually render an updated layout, browsers still need to understand changes in the DOM to provide consistent results when elements' sizes and positions are queried.

Answer 2

Your assumption is incorrect. The update of .innerHTML actually occurs synchronously (and there is no new thread created by the browser). Instead, the browser simply delays updating the window until your code has finished running. If you were to inspect the DOM in a way that necessitated an update to the view, then the browser would have to do so.

For instance, right after setting the innerHTML, include this line:

var height = element.clientHeight; // Oops, wrong... let me check again.

edit — I may discover a workaround to deceive the browser, or it could be impossible. It's definitely accurate that running your lengthy computation in a separate event loop will make it function:

setTimeout(longCalculation, 10); // Avoid using 0, especially with Firefox!

An important lesson here is that browsers strive to avoid unnecessary reflows of the page layout. If your code had taken a detour to find prime numbers and then returned to update the innerHTML once more, the browser would have saved itself from performing unnecessary work. Even if it doesn't visually render an updated layout, browsers still need to understand changes in the DOM to provide consistent results when elements' sizes and positions are queried.

Answer 3

Answer №2

From my understanding, the current code execution completes first before any page updates occur. When calling longPrimeCalc, it triggers additional code execution, and only once that is finished does the page update take place.
To resolve this issue, you need to end the current code execution and initiate the calculation in a separate context. One way to achieve this is by using setTimeout. I am not aware of any other methods to handle this situation.

Check out this jsfiddle demonstrating the behavior. You don't necessarily have to provide a callback to longPrimeCalc; instead, create another function that performs actions based on the return value. By structuring your code in this manner, it becomes clear that you are deferring the calculation to a different "thread" of execution. Here's an updated version for potential clarity:

function defer(f, callback) {
  var proc = function() {
    result = f();
    if (callback) {
      callback(result);
    }
  }
  setTimeout(proc, 50);
}

function buttonHandler() {
  var elm = document.getElementById("progress");
  elm.innerHTML = "thinking...";
  defer(longPrimeCalc, function (isPrime) {
    if (isPrime) {
      elm.innerHTML = "It was a prime!";
    }
    else {
      elm.innerHTML = "It was not a prime =(";
    }
  });
}

Answer 4

From my understanding, the current code execution completes first before any page updates occur. When calling longPrimeCalc, it triggers additional code execution, and only once that is finished does the page update take place.
To resolve this issue, you need to end the current code execution and initiate the calculation in a separate context. One way to achieve this is by using setTimeout. I am not aware of any other methods to handle this situation.

Check out this jsfiddle demonstrating the behavior. You don't necessarily have to provide a callback to longPrimeCalc; instead, create another function that performs actions based on the return value. By structuring your code in this manner, it becomes clear that you are deferring the calculation to a different "thread" of execution. Here's an updated version for potential clarity:

function defer(f, callback) {
  var proc = function() {
    result = f();
    if (callback) {
      callback(result);
    }
  }
  setTimeout(proc, 50);
}

function buttonHandler() {
  var elm = document.getElementById("progress");
  elm.innerHTML = "thinking...";
  defer(longPrimeCalc, function (isPrime) {
    if (isPrime) {
      elm.innerHTML = "It was a prime!";
    }
    else {
      elm.innerHTML = "It was not a prime =(";
    }
  });
}

The reliability of innerHTML cannot be guaranteed as it does not consistently run synchronously

Answer №1

Answer №2

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