(window.webpackJsonp=window.webpackJsonp||[]).push([[2165],{2573:function(t,s,a){"use strict";a.r(s);var n=a(31),e=Object(n.a)({},(function(){var t=this,s=t.$createElement,a=t._self._c||s;return a("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[a("h1",{attrs:{id:"asynchronous-programming"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#asynchronous-programming"}},[t._v("#")]),t._v(" Asynchronous programming")]),t._v(" "),a("p",[t._v("Node is a programming language where everything could run on an asynchronous way. Below you could find some examples and the typical things of asynchronous working.")]),t._v(" "),a("h2",{attrs:{id:"callback-functions"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#callback-functions"}},[t._v("#")]),t._v(" Callback functions")]),t._v(" "),a("h3",{attrs:{id:"callback-functions-in-javascript"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#callback-functions-in-javascript"}},[t._v("#")]),t._v(" Callback functions in JavaScript")]),t._v(" "),a("p",[t._v("Callback functions are common in JavaScript. Callback functions are possible in JavaScript because "),a("a",{attrs:{href:"https://en.wikipedia.org/wiki/First-class_function",target:"_blank",rel:"noopener noreferrer"}},[t._v("functions are first-class citizens"),a("OutboundLink")],1),t._v(".")]),t._v(" "),a("h3",{attrs:{id:"synchronous-callbacks"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#synchronous-callbacks"}},[t._v("#")]),t._v(" Synchronous callbacks.")]),t._v(" "),a("p",[t._v("Callback functions can be synchronous or asynchronous. Since Asynchronous callback functions may be more complex here is a simple example of a synchronous callback function.")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// a function that uses a callback named `cb` as a parameter")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getSyncMessage")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("cb")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("cb")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Hello World!"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\nconsole"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Before getSyncMessage call"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// calling a function and sending in a callback function as an argument.")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getSyncMessage")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("message")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    console"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("message"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\nconsole"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"After getSyncMessage call"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),a("p",[t._v("The output for the above code is:")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" Before getSyncMessage call\n"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" Hello World"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("!")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" After getSyncMessage call\n\n")])])]),a("p",[t._v("First we will step through how the above code is executed. This is more for those who do not already understand the concept of callbacks if you do already understand it feel free to skip this paragraph. First the code is parsed and then the first interesting thing to happen is line 6 is executed which outputs "),a("code",[t._v("Before getSyncMessage call")]),t._v(" to the console. Then line 8 is executed which calls the function "),a("code",[t._v("getSyncMessage")]),t._v(" sending in an anonymous function as an argument for the parameter named "),a("code",[t._v("cb")]),t._v(" in the "),a("code",[t._v("getSyncMessage")]),t._v(" function. Execution is now done inside the "),a("code",[t._v("getSyncMessage")]),t._v(" function on line 3 which executes the function "),a("code",[t._v("cb")]),t._v(' which was just passed in, this call sends an argument string "Hello World" for the param named '),a("code",[t._v("message")]),t._v(" in the passed in anonymous function. Execution then goes to line 9 which logs "),a("code",[t._v("Hello World!")]),t._v(" to the console. Then the execution goes through the process of exiting the "),a("a",{attrs:{href:"https://developer.mozilla.org/en-US/docs/Glossary/Call_Stack",target:"_blank",rel:"noopener noreferrer"}},[t._v("callstack"),a("OutboundLink")],1),t._v(" ("),a("a",{attrs:{href:"https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop",target:"_blank",rel:"noopener noreferrer"}},[t._v("see also"),a("OutboundLink")],1),t._v(") hitting line 10 then line 4 then finally back to line 11.")]),t._v(" "),a("p",[t._v("Some information to know about callbacks in general:")]),t._v(" "),a("ul",[a("li",[t._v("The function you send in to a function as a callback may be called zero times, once, or multiple times. It all depends on implementation.")]),t._v(" "),a("li",[t._v("The callback function may be called synchronously or asynchronously and possibly both synchronously and asynchronously.")]),t._v(" "),a("li",[t._v("Just like normal functions the names you give parameters to your function are not important but the order is. So for example on line 8 the parameter "),a("code",[t._v("message")]),t._v(" could have been named "),a("code",[t._v("statement")]),t._v(", "),a("code",[t._v("msg")]),t._v(", or if you're being nonsensical something like "),a("code",[t._v("jellybean")]),t._v(". So you should know what parameters are sent into your callback so you can get them in the right order with proper names.")])]),t._v(" "),a("h3",{attrs:{id:"asynchronous-callbacks"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#asynchronous-callbacks"}},[t._v("#")]),t._v(" Asynchronous callbacks.")]),t._v(" "),a("p",[t._v("One thing to note about JavaScript is it is synchronous by default, but there are APIs given in the environment (browser, Node.js, etc.) that could make it asynchronous (there's more about that "),a("a",{attrs:{href:"https://stackoverflow.com/a/13806828/2066736",target:"_blank",rel:"noopener noreferrer"}},[t._v("here"),a("OutboundLink")],1),t._v(").")]),t._v(" "),a("p",[t._v("Some common things that are asynchronous in JavaScript environments that accept callbacks:")]),t._v(" "),a("ul",[a("li",[t._v("Events")]),t._v(" "),a("li",[t._v("setTimeout")]),t._v(" "),a("li",[t._v("setInterval")]),t._v(" "),a("li",[t._v("the fetch API")]),t._v(" "),a("li",[t._v("Promises")])]),t._v(" "),a("p",[t._v("Also any function that uses one of the above functions may be wrapped with a function that takes a callback and the callback would then be an asynchronous callback (although wrapping a promises with a function that takes a callback would likely be considered an anti-pattern as there are more preferred ways to handle promises).")]),t._v(" "),a("p",[t._v("So given that information we can construct an asynchronous function similar to the above synchronous one.")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// a function that uses a callback named `cb` as a parameter")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getAsyncMessage")]),a("span",{pre:!0,attrs:{class:"token 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function"}},[t._v("getAsyncMessage")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("message")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    console"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("message"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\nconsole"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"After getSyncMessage call"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),a("p",[t._v("Which prints the following to the console:")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" Before getSyncMessage call\n"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" After getSyncMessage call\n"),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// pauses for 1000 ms with no output")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v(" Hello World"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("!")]),t._v("\n\n")])])]),a("p",[t._v('Line execution goes to line 6 logs "Before getSyncMessage call". Then execution goes to line 8 calling getAsyncMessage with a callback for the param '),a("code",[t._v("cb")]),t._v(". Line 3 is then executed which calls setTimeout with a callback as the first argument and the number 300 as the second argument. "),a("code",[t._v("setTimeout")]),t._v(" does whatever it does and holds on to that callback so that it can call it later in 1000 milliseconds, but following setting up the timeout and before it pauses the 1000 milliseconds it hands execution back to where it left off so it goes to line 4, then line 11, and then pauses for 1 second and setTimeout then calls its callback function which takes execution back to line 3 where "),a("code",[t._v("getAsyncMessages")]),t._v(' callback is called with value "Hello World" for its parameter '),a("code",[t._v("message")]),t._v(" which is then logged to the console on line 9.")]),t._v(" "),a("h3",{attrs:{id:"callback-functions-in-node-js"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#callback-functions-in-node-js"}},[t._v("#")]),t._v(" Callback functions in Node.js")]),t._v(" "),a("p",[t._v("NodeJS has asynchronous callbacks and commonly supplies two parameters to your functions sometimes conventionally called "),a("code",[t._v("err")]),t._v(" and "),a("code",[t._v("data")]),t._v(". An example with reading a file text.")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" fs "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("require")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"fs"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\nfs"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("readFile")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"./test.txt"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"utf8"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("err"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" data")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("err"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// handle the error ")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("else")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// process the file text given with data")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),a("p",[t._v("This is an example of a callback that is called a single time.")]),t._v(" "),a("p",[t._v("It's good practice to handle the error somehow even if your just logging it or throwing it. The else is not necessary if you throw or return and can be removed to decrease indentation so long as you stop execution of the current function in the if by doing something like throwing or returning.")]),t._v(" "),a("p",[t._v("Though it may be common to see "),a("code",[t._v("err")]),t._v(", "),a("code",[t._v("data")]),t._v(" it may not always be the case that your callbacks will use that pattern it's best to look at documentation.")]),t._v(" "),a("p",[t._v("Another example callback comes from the express library (express 4.x):")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// this code snippet was on http://expressjs.com/en/4x/api.html")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" express "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("require")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v("'express'")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" app "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("express")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// this app.get method takes a url route to watch for and a callback")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// to call whenever that route is requested by a user.")]),t._v("\napp"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("get")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v("'/'")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("req"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" res")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  res"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("send")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v("'hello world'")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\napp"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("listen")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("3000")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),a("p",[t._v("This example shows a callback that is called multiple times. 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