I am little bit confused about the architecture of Node.js
As i know, in browser runtime environment event loop continuously check call stack empty or not, if empty then push callback from callback queue or microtask queue into call stack.
But in nodejs, event loop checks it is blocking operation or not.
My question is that, Suppose I have a function in which first 10 lines are synchronous code on 11th line there is some asynchronous or blocking function. Then as I know (please correct me if i'm wrong), function should first go into callstack and run first 10 lines and when callstack reaches on 11th line then it will go some into another queues.
But as i attached an image link in that it is not clear, what is the flow of code execution between callstack & eventloop.
CodePudding user response：
As i know, in browser runtime environment event loop continuously check call stack empty or not, if empty then push callback from callback queue or microtask queue into call stack.
First off, there's no continuously checking the call stack. The event loop itself is at the root of the call stack. So, when user code finishes executing and returns, that return from the last bit of Javascript that was executing goes back to the event loop where it can examine the various queues and determine which event to process next.
My question is that, Suppose I have a function in which first 10 lines are synchronous code on 11th line there is some asynchronous or blocking function. Then as I know (please correct me if i'm wrong), function should first go into callstack and run first 10 lines and when callstack reaches on 11th line then it will go some into another queues.
An asynchronous function does NOT block. It starts some operation and then code just keeps on executing. So, as an example, if you call fs.readFile() or fs.promises.readFile() in nodejs, those function calls initiate the operation and then immediately return and the line of code right after that executes. As this code after your call to .readFile() finishes executing, it will eventually return control back to the event loop where, some time in the future, that .readFile() function will complete and insert an event in an event queue that the event loop can then process.
what is the flow of code execution between callstack & eventloop.
The call stack controls a given chunk of Javascript executing. When a function call is made, the place that execution should continue at when that function returns is pushed into the callstack. It's just a mechanism for handling function calls. Unlike other languages, variables scoped within a function are not put on the stack because those type of scope variables in Javascript can have a different lifetime than the execution of the function. They are garbage collected instead. So, the callstack in Javascript is just return locations from function calls.
Because nodejs runs your Javascript as a single thread, the event loop does not get to execute any waiting events until a given chunk of Javascript finishes executing, the call stack is then empty and control returns back to the event loop where it can then start a new chunk of Javascript executing to process the next event.
So, the callstack facilitates running a chunk of Javascript. Only when the callstack is empty and control returns back to the event loop can the next event waiting to be processed get run. The two do very different things.