Node-JS Asynchronous and Single-threaded [NOTE]

Blocking and Non-Blocking with Node.JS

• Blocking methods execute synchronously and non-blocking methods execute asynchronously.

  • Example:

     // This is a synchronous file read.
     const fs = require('fs');
     const data = fs.readFileSync('/file.md'); // blocks here until file is read

     // This is a asynchronous
     const fs = require('fs');
     fs.readFile('/file.md', (err, data) => {
       if (err) throw err;
     });

     const fs = require('fs');
     const data = fs.readFileSync('/file.md'); // blocks here until file is read
     console.log(data);
     moreWork(); // will run after console.log

     const fs = require('fs');
     fs.readFile('/file.md', (err, data) => {
       if (err) throw err;
       console.log(data);
     });
     moreWork(); // will run before console.log

Single Thread

• Javascript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work.
• JavaScript is an interpreted language, not a compiled one.
• There are several types of interpreters (known as engines). The most popular browser engines are V8 (Chrome), Quantum (Firefox) and WebKit (Safari).
• A single-thread language is one with a single call stack and a single memory heap. It means that it runs only one thing at a time.
• A stack is a continuous region of memory, allocating local context for each executed function.
• A heap is a much larger region, storing everything allocated dynamically.

• As you can see, the functions are added to the stack, executed and later deleted. It's the so-called LIFO way - Last In, First Out. Each entry in the call stack is called a stack frame.

• However, JS can also be non-blocking and behave as if it were multi-threaded. It means that it doesn't wait for the response of an API call, I/O events, etc., and can continue the code execution. It is possible thanks to the JS engines which use (under the hood) real multi-threading languages, like C++ (Chrome) or Rust (Firefox). They provide us with the Web API under the browser hoods or ex. I/O API under Node.js.

  

Note

• You can try call-stack structure. Loupe

Thank you

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