C++ Essay

var id = 2;
function x() {
  console.log(id);  //reference error 
  let id = 3;
  console.log(id); 
}
x();

雖然let引入block scope，避免了var的variable hoisting，但是須注意

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let

let bindings are created at the top of the (block) scope containing the declaration, commonly referred to as "hoisting". Unlike variables declared with var, which will start with the value undefined, let variables are not initialized until their definition is evaluated. Accessing the variable before the initialization results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the initialization is processed.

所以上面的id會引發reference error
但是比較var範例

var id = 2;
function x() {
  console.log(id); 
  //undefined，因為下面定義的variable hoisting
  var id = 3;
  console.log(id); 
}
x();

JSON5 (https://spec.json5.org)

https://github.com/json5/json5

nodejs讀取設定檔常使用json格式(http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf, https://tools.ietf.org/html/rfc8259)， 但是json在描述時限制較多，例如在key要雙引號，字串要用雙引號不能用單引號，不能有註解等。

因此可讀性變差，JSON5主要將ECMAScript 5.1語法引入，整體看來，syntax對於熟悉javascript的使用者更友善

幾個比較大的差異

• key不一定需要引號(根據ECMAScript 5.1 IdentifierName規則)
• Objects may have a single trailing comma.
• Arrays may have a single trailing comma.
• Strings may be single quoted/支援多行/支援 \x的escape (U+0000 through U+00FF)
• String更寬容的escape字元包含
  • JSON:All Unicode characters may be placed within the
    quotation marks, except for the characters that MUST be escaped: quotation mark, reverse solidus, and the control characters (U+0000 through U+001F).
  • JSON5:All Unicode characters may be placed within the quotation marks, except for the characters that must be escaped: the quotation mark used to begin and end the string, reverse solidus, and line terminators.
• Number支援更廣泛的表示方式(eg. hex, + sign), inf, NaN
• Single and multi-line comments are allowed
• 更寬容的whitespace支援 (JSON只支援0x20, 0x09, 0x0A, 0x0D)
   

https://www.npmjs.com/package/rootpath

//BEFORE

// from $HOME_PROJECT/lib/math/
var myLib = require('../myLibrary');
var myUtils = require('../../utils/myUtils');
var myTest = require('../../test/myTest');
//AFTER

// from $HOME_PROJECT/lib/math/

require('rootpath')();

var myLib = require('lib/myLibrary');
var myUtils = require('utils/myUtils');
var myTest = require('test/myTest');

在vsphere rest sdk sample看到的一個用法，這對於library更動目錄時不需要再改寫require path非常方便，但是須注意webpack使用時，應該用webpack resolve configuration來處理

在需要呼叫多次callback時，async await可改善callback hell的現象
而nodejs在8.x支援原生的async await，在設計callback api時，可設計成同時支援舊的callback style與新的promise style，這樣一來可以使api同時支援’async’ library與async/await語法

作法1:

function delay(ms, cb){
  var func = delay;
  if(cb === undefined){
    //檢查是否有callback，如果沒有返回promise 
    return new Promise(function(resolve, reject){
      func(ms, function(err, data){
        if(err) return reject(err);
        resolve(data);
      });
    });
  }
  setTimeout(function(){ cb(null, Date.now()); }, ms); 
}

作法2: 參考: https://zaiste.net/node_js_functions_simultaneously_supporting_callbacks_promises/

function delay(ms, cb = function(){}){
  var func = delay;
  return new Promise(function(resolve, reject){
    setTimeout(function(){
      resolve(Date.now());
      cb(null, Date.now());
    }, ms);
  });
}

使用:

//callback用法
delay(2000, function(err, data){
  if(err){
    console.log('error', err);
  } else {
    console.log(data);
  }
});
//promise then用法
delay(2000).then(data => console.log(data)).catch(err => console.log('error', err));

//async await用法，await要包在async function內
(async function(){
  try{
    var resp = await delay(2000); //resolve時才往下走
    console.log(resp);
  }catch(e){
    console.log('error', e);
  }
})();

在 https://github.com/expressjs/express/blob/master/lib/router/index.js#L46

function router(req, res, next) {
router.handle(req, res, next);
}

express router不會catch error，所以error handler定義在router內無法作用
要直接定義在app那層，參考doc Error-handling middleware

app.use(function(err, req, res, next) {
console.error(err.stack);
res.status(500).send('Something broke!');
});


另外是順序也很重要，next(err)會依照順序往下找，因此error handler應該定義在最後面

參考來源:
https://stackoverflow.com/questions/45431595/express-error-handler-inside-router-doesnt-work