Stanford CS142 笔记 - Node.js

Introduction

Node.js is an open-source, cross-platform JavaScript runtime environment that executes JavaScript code outside of a web browser.

• Single-Threaded Event Loop: Node.js uses a single-threaded event loop. The Event Loop continuously checks the event queue to see if there are any events to be handled.
• Non-blocking I/O: Node.js performs I/O operations (like reading from a database or reading from a file) asynchronously. Instead of waiting for the operation to complete, Node.js registers a callback and moves on to handle the next event. This non-blocking behavior ensures that Node.js can handle many I/O operations concurrently without getting bogged down by slow operations

Module System

• require is a function in Node.js used to load modules. We use Express and Mongoose for web development.
  • System modules: Built-in Node.js modules like fs, http, path, etc.
  • File modules: Like require("./myModule.js")
  • Directory modules: Like require("./myModule"), look for myModule/index.js
• In Node.js, every module (or file) has its own scope, module.exports allows us to make functions or variables available to other files.

  var notGlobal;
  function func1() {}
  function func2() {}
  module.exports = {func1: func1, func2: func2};

Non-blocking

r1 = step1();
console.log('step1 done', r1);
r2 = step2(r1);
console.log('step2 done', r2);
r3 = step3(r2);
console.log('step3 done', r3);
console.log('All Done!');

If step() function doesn’t block, then it’s ok.

step1(function (r1) {
    console.log('step1 done', r1);
    step2(r1, function (r2) {
        console.log('step2 done', r2);
        step3(r2, function (r3) {
            console.log('step3 done', r3);
            console.log('All Done!');  // Correct
        });
    });
});
console.log('All Done!'); // Wrong!

If the function is blocking, use callbacks.

Listener and Emitter

myEmitter.on('myEvent', function(param1, param2) {
  console.log('myEvent occurred with ' + param1 + ' and ' + param2 + '!');
});
myEmitter.emit('myEvent', 'arg1', 'arg2');

On emit call listeners are called synchronously and in the order the listeners were registered.

Have multiple different events for different state or actions:

myEmitter.on('conditionA', doConditionA);
myEmitter.on('conditionB', doConditionB);
myEmitter.on('conditionC', doConditionC);
myEmitter.on('error', handleErrorCondition);