JavaScript is single threaded, that means only one statement is executed at a time. As the JS engine processes our script line by line, it uses this single Call-Stack to keep track of codes that are supposed to run in their respective order.
Like what a stack does, a data structure which records lines of executable instructions and executes them in LIFO manner. So say if the engine steps into a function foo(){ it PUSH-es foo() into the stack and when the execution of foo()return; } is over foo() is POP-ped out of the call-stack.
EXERCISE 1: So from the above diagram shows how a typical line by line execution happens. When the script of three console.log() statements is thrown at JS —
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Step 1: The console.log("Print 1") is pushed into the call stack and executed, once done with execution, it is then popped out of the stack. Now the stack is empty and ready for any next instruction to be executed.
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Step 2: console.log("Print 2"); // is the next instruction is pushed and the same thing repeats until
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Step 3: is executed and there is nothing left to push and execute.
Lets take an example to understand it better, its a famous Interview question as well. In the logText() function, I am only doing console.log() three times.
const logText = () => {
console.log('A');
setTimeout(() => {
console.log('B');
}, 0);
console.log('C');
}
logText()The output A C B
Explanation - Even though, I am passing 0 to the setTimeout() for the waiting period, because setTimeout() invokes a callback function - this callback function will be passed to an Event Loop. And we we know that - The event loop as a process is a set of phases with specific tasks that are processed in a round-robin manner - so the callback function will go through those phases and ultimately get executed. But during this process Node being Node, will be non-blocking, and so would NOT block the next lines of code which is the third line < console.log('C') >
Worker pool is an execution model that spawns and handles separate threads, which then synchronously perform the task and return the result to the event loop. The event loop then executes the provided callback with said result.