5 ms·

naive question: What is implementation difference between async and synchronous function that makes many languages differentiate them?

shadowgovt 7y ago
I'm not sure if you're talking about the literal `async` keyword in JavaScript or the general concept of asynchronous functions. I'll answer both questions. Declaring a function `async` in JavaScript causes the function to return a Promise immediately instead of executing all the code within the function (and blocking until that execution is completed). A Promise can be thought of as a "placeholder" for data not available yet; something can later put data into it (which will trigger all code waiting on the promise to have data to be executed). If you call another async function within an async function, you're allowed to use the keyword "await" to block execution of the calling function until the called function either satisfies or fails the returned Promise, at which point the return Promise result is used as the return value of the awaited function and the waiting function finished execution. More generally (speaking loosely), synchronous functions are evaluated to completion before the next line of code is run, and async functions return immediately but either queue up some work to be done later or kick off some work in a separate system (additional thread, additional processor, maybe the network card or graphics card) that happens at the same time your main program body is running. So, for example, "Math.pow(3,3)" is synchronous and your code won't continue until three-cubed is calculated, but "XmlHttpRequeest.open" with its async parameter set (https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/open https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequ...) will return immediately and let more of your JavaScript code run while the browser itself communicates with the network software / hardware to make an HTTP request, fetch some data, and give it to your program; you can register a function to be run once that data is available. ADDENDUM: WHY WE CARE SO MUCH It's generally impossible (I'm lying, but it's a useful lie) for completely synchronous code to "deadlock," which is where there is no way for the next line of code to be run. With asynchronous code, deadlock happens when an asynchronous function is waiting on some result that will never come; the program cannot continue, and (if it blocks your user interface) your computer is frozen. Without some discipline of ordering of events or ownership of task responsibility, deadlock is very easy to cause with asynchronous code. Trivial deadlock example: thread A: start async thread B, wait for a '3' from B, then send '5' to B thread B: wait for a '5' from A, then send '3' to A These threads get stuck forever; A is waiting for B to give it a '3', and B is waiting for A to give it a '5'.

shadowgovt 7y ago

I'm not sure if you're talking about the literal `async` keyword in JavaScript or the general concept of asynchronous functions. I'll answer both questions. Declaring a function `async` in JavaScript causes the function to return a Promise immediately instead of executing all the code within the function (and blocking until that execution is completed). A Promise can be thought of as a "placeholder" for data not available yet; something can later put data into it (which will trigger all code waiting on the promise to have data to be executed). If you call another async function within an async function, you're allowed to use the keyword "await" to block execution of the calling function until the called function either satisfies or fails the returned Promise, at which point the return Promise result is used as the return value of the awaited function and the waiting function finished execution. More generally (speaking loosely), synchronous functions are evaluated to completion before the next line of code is run, and async functions return immediately but either queue up some work to be done later or kick off some work in a separate system (additional thread, additional processor, maybe the network card or graphics card) that happens at the same time your main program body is running. So, for example, "Math.pow(3,3)" is synchronous and your code won't continue until three-cubed is calculated, but "XmlHttpRequeest.open" with its async parameter set (https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/open https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequ...) will return immediately and let more of your JavaScript code run while the browser itself communicates with the network software / hardware to make an HTTP request, fetch some data, and give it to your program; you can register a function to be run once that data is available. ADDENDUM: WHY WE CARE SO MUCH It's generally impossible (I'm lying, but it's a useful lie) for completely synchronous code to "deadlock," which is where there is no way for the next line of code to be run. With asynchronous code, deadlock happens when an asynchronous function is waiting on some result that will never come; the program cannot continue, and (if it blocks your user interface) your computer is frozen. Without some discipline of ordering of events or ownership of task responsibility, deadlock is very easy to cause with asynchronous code. Trivial deadlock example: thread A: start async thread B, wait for a '3' from B, then send '5' to B thread B: wait for a '5' from A, then send '3' to A These threads get stuck forever; A is waiting for B to give it a '3', and B is waiting for A to give it a '5'.