调用另一个回调 function 的 setInterval 回调不起作用

Question

Here is my code

function doIt(context, startTime) {
    context.url((currentURL) => {
        console.log("URL: " + currentURL.value)
    })
    console.log("running timer function ")
    if (new Date() - startTime > 5000) { // timeout = 5 seconds
        console.log("timed out")
        clearInterval(timerId) // exit the loop
    } else {
        console.log("keep on truckin'")
    }
}

let timerId = setInterval(doIt, 1000, this, Date.now())

Instead of looping five times (once per second for 5 seconds), it is only running once and the output comes out as...

running timer function
keep on truckin'
URL: https://michigan.magellanrx.com/

If I comment out the first 3 lines of code (the context.url function) then the code runs as expected with the output...

running timer function
keep on truckin'
running timer function
keep on truckin'
running timer function
keep on truckin'
running timer function
keep on truckin'
running timer function
timed out

Why isn't the code running through all five iterations when the context.url function isn't commented out?

Clarification on this... it turns out it's not running just once. Indeed, what is happening is that the setInterval function is changing from a "blocking" function to a "non-blocking" function when those first 3 lines of code are included. So, instead of running through all five iterations before moving on, it's only running the first iteration then moving on to subsequent commands while the setInterval continues to run in the background.

Answer 1

From your description of the symptom it sounds like you may have an infinite loop in context.url() .

Infinite loops blocks the main thread, thereby causing no events to be processed. You need to find and remove that infinite loop.

---

How javascript is implemented in C/C++

The way the javascript interpreter is implemented is usually something like the following:

// pseudocode:
do {
    eventHandlers = executeJavascript();
    // end of execution

    events = waitForAllIO(); // this actually blocks but it is waiting
                             // for ALL I/O instead of just one

    if (events.timeout) {
        foreach (callback from eventHandlers) {
            if (callback is TIMEOUT_HANDLER) {
                callback(events.timeout);
            }
        }
    }
    else {
        foreach (event from events) {
             foreach (callback from eventHandlers) {
                 if (callback is for event) {
                     callback(event);
                 }
             }
        }
    }
} while (eventHandlers.length > 0)

If your code is executing an infinite loop, that means the interpreter gets stuck at the executeJavascript() part. That means no timers or.network packets or any other events gets processed. Which explains why your setInterval only produces an output once.