Nodejs小知识之event（错误事件和仅处理事件一次）三

event - 错误事件和仅处理事件一次

当使用 eventEmitter.on() 注册监听器时，监听器会在每次触发命名事件时被调用。

const EventEmitter = require("events");

const emitter = new EventEmitter();

emitter.on("event", function () {
  console.log("event emitter");
});

emitter.emit("event");
emitter.emit("event");

运行后得到的结果如下

$ node main.js
event emitter
event emitter

使用 eventEmitter.once() 可以注册最多可调用一次的监听器。当事件被触发时，监听器会被注销，然后再调用。

const EventEmitter = require("events");

const emitter = new EventEmitter();

emitter.once("event", function () {
  console.log("event emitter");
});

emitter.emit("event");
emitter.emit("event");

运行后得到的结果如下

$ node main.js
event emitter

（错误事件）Error events

const EventEmitter = require("events");

const emitter = new EventEmitter();

emitter.emit("error", new Error("i am a error"));

运行结果如下

$ node main.js
events.js:174
      throw er; // Unhandled 'error' event
      ^

Error: i am a error
    at Object.<anonymous> (/Users/durban/nodejs/main.js:45:23)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:623:3)
Emitted 'error' event at:
    at Object.<anonymous> (/Users/durban/nodejs/main.js:45:9)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    [... lines matching original stack trace ...]
    at bootstrapNodeJSCore (internal/bootstrap/node.js:623:3)

再看下面实例

const EventEmitter = require("events");

const emitter = new EventEmitter();

emitter.on("error", function (error) {
  console.log("i am catch a error");
  console.log(error);
});

emitter.emit("error", new Error("i am a error"));

运行后结果如下

$ node main.js
i am catch a error
Error: i am a error
    at Object.<anonymous> (/Users/durban/nodejs/main.js:77:23)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:623:3)

event（事件）的简单使用记录再次