Const / Let

Variable Declarations with Const and Let

ES6 主要有兩個方向的變革，一個是類似之前課程所提到的一系列 語法糖（syntax sugar），可以使開發過程中儘可能減少代碼量來增加開發效率，另一個變革則是讓語言在開發過程中更加穩健。這一小節所要提及的變數宣告關鍵字 const 和 let 就屬於第二種。

// const and let
const name = 'Jane';
let title = 'Soft Engineer';
let hourlyWage = 40;

title = 'Senior Soft Engineer';
hourlyWage = 45;

關鍵字 const 用以宣告變數，宣告之後不能改變變數型態及其所儲存的值。

What Const and Let Solve

ES6 中使用 const 和 let 作為宣告變數的關鍵字，最主要是在處理使用 var 可能延伸出的種種問題，可以參考 2017 iT 邦鐵人賽 | Day 05: ES6 篇 - let 與 const 這篇文章中的講解會比講師說得更為詳細一些。

[Exercise] Letting Variables Be Variables

Question

Imagine that your are building an application to manage a user's Facebook profile. A profile might have a name, age and dateOfBirth. Declare three variables with these same names, making sure to use const or let depending on whether you expect the value to chnage over time.

Solution

const name = 'Peter';
const dateOfBirth = '900422';
let age = 25;

[Exercise] Const/Let Refactoring

Question

The following code uses var instead of const and let. Refactor the function to use the new keywords. Be sure to consider whether the variable should be declared using const or let depending on whether the variable gets reassigned or not.

var statuses = [
  { code: 'OK', response: 'Request successful' },
  { code: 'FAILED', response: 'There was an error with your request' },
  { code: 'PENDING', response: 'Your reqeust is still pending' }
];
var message = '';
var currentCode = 'OK';

for (var i = 0; i < statuses.length; i++) {
  if (statuses[i].code === currentCode) {
    message = statuses[i].response;
  }
}

Solution

const statuses = [
  { code: 'OK', response: 'Request successful' },
  { code: 'FAILED', response: 'There was an error with your request' },
  { code: 'PENDING', response: 'Your reqeust is still pending' }
];
let message = '';
let currentCode = 'OK';

for (let i = 0; i < statuses.length; i++) {
  if (statuses[i].code === currentCode) {
    message = statuses[i].response;
  }
}