feat(curriculum): introduce let and const earlier (#43133)

* fix: move "Explore Differences Between..." to basic JS, update seed and tests

* fix: resequence "Declare String Variables"

* fix: move "Declare a Read-Only Variable..." to basic JS, update seed and tests

* fix: revert changes to non-English "Explore Differences Between..." test text

* fix: revert test strings, solutions, and seeds for non-English challenges

* fix: update "Declare String Variables" description

* fix: sync quotation marks in description and seed

* fix: modify note in "Declare a Read-Only..." challenge

* fix: update operator and compound assignment challenges

* fix: update string challenges

* fix: update array and array method challenges

* fix: update function and scope challenges, resequence slightly

* fix: "Word Blanks" solution

* fix: add spacing to seed

* fix: concatenating += challenge spacing

* fix: appending variables to strings spacing

* fix: find the length of a string spacing

* fix: removed instances of removedFromMyArray = 0

* fix: switch challenges

* fix: function argument and param spacing

* fix: update counting cards, object challenges, and record collection

* fix: finish rest of Basic JS section

* fix: introducing else statements solution

* fix: update spacing and wording

* fix: update wording for const challenge

* fix: update functional programming challenges

* fix: intermediate algorithms and cert challenges

* fix: revert some spacing and remove comments for fp challenge solutions

* feat: add notes with links to moved let and const challenges in first two es6 challenges

* fix: update es6 intro text

* Update curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-javascript/concatenating-strings-with-the-plus-equals-operator.md

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* Update curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-javascript/finding-a-remainder-in-javascript.md

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* Update curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-javascript/global-scope-and-functions.md

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* Update curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-javascript/iterate-through-an-array-with-a-for-loop.md

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* Update curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-javascript/iterate-through-an-array-with-a-for-loop.md

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* Update curriculum/challenges/english/02-javascript-algorithms-and-data-structures/functional-programming/implement-map-on-a-prototype.md

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* Update curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-javascript/declare-a-read-only-variable-with-the-const-keyword.md

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* fix: concatenating strings with plus operator seed

* fix: add comments back to Declare a Read-Only Variable... seed

* feat: add es6 to basic javascript redirect tests for let and const challenges

* fix: revert "Concatenating Strings with Plus Operator" seed

* fix: move test file to cypress/integration/learn/redirects, separate redirect tests

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

curriculum/challenges/chinese/02-javascript-algorithms-and-data-structures/basic-javascript/declare-a-read-only-variable-with-the-const-keyword.md

@@ -0,0 +1,89 @@
+---
+id: 587d7b87367417b2b2512b41
+title: 用 const 关键字声明只读变量
+challengeType: 1
+forumTopicId: 301201
+dashedName: declare-a-read-only-variable-with-the-const-keyword
+---
+
+# --description--
+
+`let` 并不是唯一的新的声明变量的方式。 在 ES6 里面，你还可以使用 `const` 关键字来声明变量。
+
+`const` 拥有 `let` 的所有优点，不同的是，通过 `const` 声明的变量是只读的。 这意味着通过 `const` 声明的变量只能被赋值一次，而不能被再次赋值。
+
+```js
+const FAV_PET = "Cats";
+FAV_PET = "Dogs";
+```
+
+控制台将由于给 `FAV_PET` 重新赋值而显示错误。
+
+可见，尝试给用 `const` 声明的变量重新赋值会报错。 你应该使用 `const` 关键字来声明所有不打算再次赋值的变量。 这有助于避免给一个常量进行额外的再次赋值。 一个最佳实践是对所有常量的命名采用全大写字母，并在单词之间使用下划线进行分隔。
+
+**注意：**通常，开发者会用大写字母作为常量标识符，用小写字母或者驼峰命名作为变量（对象或数组）标识符。 后面的挑战会涉及到在数组中使用小写变量标识符。
+
+# --instructions--
+
+改变以下代码，使得所有的变量都使用 `let` 或 `const` 关键词来声明。 当变量将会改变的时候使用 `let` 关键字，当变量要保持常量的时候使用 `const` 关键字。 同时，对使用 `const` 声明的变量按照最佳实践重命名，变量名中的字母应该都是大写的。
+
+# --hints--
+
+代码中不应有 `var`。
+
+```js
+(getUserInput) => assert(!getUserInput('index').match(/var/g));
+```
+
+`SENTENCE` 应该是使用 `const` 声明的常量。
+
+```js
+(getUserInput) => assert(getUserInput('index').match(/(const SENTENCE)/g));
+```
+
+`i` 应该是使用 `let`声明的。
+
+```js
+(getUserInput) => assert(getUserInput('index').match(/(let i)/g));
+```
+
+`console.log` 应该修改为用于打印 `SENTENCE` 变量。
+
+```js
+(getUserInput) =>
+  assert(getUserInput('index').match(/console\.log\(\s*SENTENCE\s*\)\s*;?/g));
+```
+
+# --seed--
+
+## --seed-contents--
+
+```js
+function printManyTimes(str) {
+
+  // Only change code below this line
+
+  var sentence = str + " is cool!";
+  for (var i = 0; i < str.length; i+=2) {
+    console.log(sentence);
+  }
+
+  // Only change code above this line
+
+}
+printManyTimes("freeCodeCamp");
+```
+
+# --solutions--
+
+```js
+function printManyTimes(str) {
+
+  const SENTENCE = str + " is cool!";
+  for (let i = 0; i < str.length; i+=2) {
+    console.log(SENTENCE);
+  }
+
+}
+printManyTimes("freeCodeCamp");
+```

curriculum/challenges/chinese/02-javascript-algorithms-and-data-structures/basic-javascript/explore-differences-between-the-var-and-let-keywords.md

@@ -0,0 +1,91 @@
+---
+id: 587d7b87367417b2b2512b3f
+title: 探索 var 和 let 关键字之间的差异
+challengeType: 1
+forumTopicId: 301202
+dashedName: explore-differences-between-the-var-and-let-keywords
+---
+
+# --description--
+
+使用 `var` 关键字来声明变量，会出现重复声明导致变量被覆盖却不会报错的问题。
+
+```js
+var camper = 'James';
+var camper = 'David';
+console.log(camper);
+```
+
+这里控制台将显示字符串 `David`。
+
+在上面的代码中，`camper` 变量的初始值为 `James`，然后又被覆盖成了 `David`。 在小型的应用中，你可能不会遇到这样的问题。但是当你的代码规模变得更加庞大的时候，就可能会在不经意间覆盖了之前定义的变量。 因为这样的情况不会报错，所以搜索和修复 bug 会变得非常困难。  
+在 ES6 中引入了新的关键字 `let` 来解决 `var` 关键字带来的潜在问题。 如果你在上面的代码中使用 `let` 关键字来代替 `var` 关键字，结果会是一个报错。
+
+```js
+let camper = 'James';
+let camper = 'David';
+```
+
+你可以在浏览器的控制台里看见这个错误。 与 `var` 不同的是，当使用 `let` 的时候，同一名字的变量只能被声明一次。 请注意 `"use strict"`。 这代表着开启了严格模式，用于检测常见的代码错误以及“不安全”的行为， 例如：
+
+```js
+"use strict";
+x = 3.14;
+```
+
+这将显示一个错误 `x is not defined`。
+
+# --instructions--
+
+请更新这段代码，只使用 `let` 关键字。
+
+# --hints--
+
+代码中不应有 `var`
+
+```js
+(getUserInput) => assert(!getUserInput('index').match(/var/g));
+```
+
+`catName` 变量的值应该为 `Oliver`
+
+```js
+assert(catName === 'Oliver');
+```
+
+`quote` 变量的值应该为 `Oliver says Meow!`
+
+```js
+assert(quote === 'Oliver says Meow!');
+```
+
+# --seed--
+
+## --seed-contents--
+
+```js
+var catName;
+var quote;
+function catTalk() {
+  "use strict";
+
+  catName = "Oliver";
+  quote = catName + " says Meow!";
+
+}
+catTalk();
+```
+
+# --solutions--
+
+```js
+let catName;
+let quote;
+function catTalk() {
+  'use strict';
+
+  catName = 'Oliver';
+  quote = catName + ' says Meow!';
+}
+catTalk();
+```