I have been lately reading about the differences between macro and micro tasks in JavaScript. I stumbled upon a thing I can't understand. In this article there is an example of promise usage in coditionals and how they can affect execution ordering. Here is a short snippet I did to reproduce it on my PC:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<div id="root"></div>
<script>
const root = document.getElementById("root");
const cache = {};
root.addEventListener("load", () => console.log("Loaded data"));
function getData(url) {
if (cache[url]) {
root.dispatchEvent(new Event("load"));
} else {
Promise.resolve({a: 5}).then(data => {
cache[url] = data;
root.dispatchEvent(new Event("load"));
});
}
}
</script>
</body>
</html>
Having the above HTML page open in the browser pase the following code in the console two times.
console.log("Fetching data...");
getData("test");
console.log("Data fetched");
The result of this is the following:
Fetching data...
Data fetched
Loaded data
Fetching data...
Loaded data
Data fetched
I can't quite get why the order of the log statements is different the second time I execute the code. From what I understand micro/macro tasks can't be processed before the current script or task in this case (as far as I know every snippet of code is wrapped in a macrotask which is then processed by the event loop) finishes. I would expect the same order in all cases.
question from:
https://stackoverflow.com/questions/65600938/microtask-executed-before-the-current-task-finishes 与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
