Blocking calls are ones in which more JavaScript must wait until a non-JavaScript action (such as anything with input or output) completes or ends before continuing. Consider this to be a synchronous activity. Non-blocking calls can run asynchronously, giving them a performance benefit.  This is significant because JavaScript is single-threaded, which means it runs code in a specified order, and each action must complete before proceeding to the next. There is just one call stack and one memory heap in JavaScript. The JavaScript engine can assist in the processing of asynchronous code in the browser.  Most I/O methods in Node.js provide both synchronous and asynchronous options. The fs.readFileSync function is an example of a forced synchronous file read, whereas the fs.readFile method is an example of an asynchronous file read.