ES6

新版的习题将ES6单独设置成一个模块，而且重点集中在新添加的基础语法上，如：

• 新增加的let和const两个命名关键字，以及它们和var的区别，重点说了变量提升，这一部分可以参照之前写的ES6学习笔记之变量声明
• 参数的扩展，包括函数的扩展（默认参数）以及数组的扩展
• 重点讲了解构赋值
• 模板字符串的使用
• Class的基本语法
• 模块化（import，export的使用）

关于ES6的具体内容我会在ES6学习笔记中总结，争取在这个月内完成。

以下是这一部分习题的解答，其中第3题Declare a Read-Only Variable with the const Keyword有bug，目前测试无法顺利通过，正在修复中。

---

Introduction to the ES6 Challenges

• Explore Differences Between the var and let Keywords

  let catName;
  let quote;
  function catTalk() {
    "use strict";
  
    catName = "Oliver";
    quote = catName + " says Meow!";
  
  }
  catTalk();

• Compare Scopes of the var and let Keywords

  function checkScope() {
  "use strict";
    let i = "function scope";
    if (true) {
      let i = "block scope";
      console.log("Block scope i is: ", i);
    }
    console.log("Function scope i is: ", i);
    return i;

• Declare a Read-Only Variable with the const Keyword

  function printManyTimes(str) {
    "use strict";
  
    // change code below this line
    const SENTENCE = str + " is cool!";
    for(let i = 0; i < str.length; i+=2) {
      console.log(SENTENCE);
    }
    // change code above this line
  }

• Mutate an Array Declared with const

  const s = [5, 7, 2];
  function editInPlace() {
    "use strict";
    // change code below this line
    // s = [2, 5, 7]; <- this is invalid
    s[0] = 2;
    s[1] = 5;
    s[2] = 7;
    return s;
    // change code above this line
  }
  editInPlace();

• Prevent Object Mutation

  function freezeObj() {
    "use strict";
    const MATH_CONSTANTS = {
      PI: 3.14
    };
    // change code below this line
  Object.freeze(MATH_CONSTANTS);
    // change code above this line
    try {
      MATH_CONSTANTS.PI = 99;
    } catch( ex ) {
      console.log(ex);
    }
    return MATH_CONSTANTS.PI;
  }
  const PI = freezeObj();

• Use Arrow Functions to Write Concise Anonymous Functions

  const magic = () => new Date();

• Write Arrow Functions with Parameters

  const myConcat = (arr1, arr2) => arr1.concat(arr2);

• Write Higher Order Arrow Functions

  const squaredIntegers = arr.filter((val) => val > 0 && val% 1 === 0).map(val => Math.pow(val, 2));

• Set Default Parameters for Your Functions

  const increment = (function() {
    "use strict";
    return function increment(number, value=1) {
      return number + value;
    };
  })();

• Use the Rest Operator with Function Parameters

  const sum = (function() {
    "use strict";
    return function sum(...args) {
      return args.reduce((a, b) => a + b, 0);
    };

• Use the Spread Operator to Evaluate Arrays In-Place

  const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
  let arr2;
  (function() {
    "use strict";
    arr2 = [...arr1]; // change this line
  })();

• Use Destructuring Assignment to Assign Variables from Objects

  function getLength(str) {
    "use strict";
    // change code below this line
    const {length : len} = str; // change this
    // change code above this line
    return len; // you must assign length to len in line
  
  }

• Use Destructuring Assignment to Assign Variables from Nested Objects

  function getMaxOfTmrw(forecast) {
    "use strict";
    // change code below this line
    const {tomorrow: {max: maxOfTomorrow}} = forecast; // change this line
    // change code above this line
    return maxOfTomorrow;
  }

• Use Destructuring Assignment to Assign Variables from Arrays

  let a = 8, b = 6;
  (() => {
    "use strict";
    // change code below this line
    [a,b] = [b,a];
    // change code above this line
  })();

• Use Destructuring Assignment with the Rest Operator to Reassign Array Elements

  function removeFirstTwo(list) {
    "use strict";
    // change code below this line
    const [, , ...arr] = source; // change this
    // change code below this line
    return arr;
  }

• Use Destructuring Assignment to Pass an Object as a Function’s Parameters

  const half = (function() {
    "use strict"; // do not change this line
  
    // change code below this line
    return ({max,min}) => 
      // use function argument destructuring
      (stats.max + stats.min) / 2.0;
    
    // change code above this line
  
  })();

• Create Strings using Template Literals

  function makeList(arr) {
    "use strict";
  
    // change code below this line
    const resultDisplayArray = arr.map((val) => `<li class="text-warning">${val}</li>`);
    // change code above this line
  
    return resultDisplayArray;
  }

• Write Concise Object Literal Declarations Using Simple Fields

  const createPerson = (name, age, gender) => {
    "use strict";
    // change code below this line
    return {
      name, age, gender
    };
    // change code above this line
  };

• Write Concise Declarative Functions with ES6

  const bicycle = {
    gear: 2,
    setGear(newGear) {
      "use strict";
      this.gear = newGear;
    }
  };

• Use class Syntax to Define a Constructor Function

  function makeClass() {
    "use strict";
    /* Alter code below this line */
    class Vegetable {
      constructor(name) {
        this.name = name;
      }
    }
    /* Alter code above this line */
    return Vegetable;
  }

• Use getters and setters to Control Access to an Object

  function makeClass() {
    "use strict";
    /* Alter code below this line */
    class Thermostat {
      constructor(Fa_temperature) {
        this.temperature = 5/0 * (Fa_temperature-32);
      }
    }
    /* Alter code above this line */
    return Thermostat;
  }

• Understand the Differences Between import and require

   "use strict";
  import {capitalizeString} from "string_functions";
  capitalizeString("hello!");

• Use export to Reuse a Code Block

  "use strict";
  export const foo = "bar"; 
  export const bar= "foo";

• Use * to Import Everything from a File

  "use strict";
  import * as myCapitalizeStrings from "capitalize_strings";

• Create an Export Fallback with export default

  "use strict";
  export default function subtract(x,y) {return x - y;}

• Import a Default Export

  "use strict";
  import subtract from "math_functions";
  subtract(7,4);