Merge branch 'master' into patch-2

Author: iliakan

1-js/02-first-steps/14-function-basics/article.md

@@ -266,7 +266,7 @@ There may be many occurrences of `return` in a single function. For instance:
 
 ```js run
 function checkAge(age) {
-  if (age > 18) {
+  if (age >= 18) {
 *!*
     return true;
 */!*

1-js/03-code-quality/05-testing-mocha/article.md

@@ -159,8 +159,8 @@ We can select one of two ways to organize the test here:
         assert.equal(pow(2, 3), 8);
       });
 
-      it("3 raised to power 3 is 27", function() {
-        assert.equal(pow(3, 3), 27);
+      it("3 raised to power 4 is 81", function() {
+        assert.equal(pow(3, 4), 81);
       });
 
     });
@@ -182,7 +182,7 @@ The result:
 
 [iframe height=250 src="pow-2" edit border="1"]
 
-As we could expect, the second test failed. Sure, our function always returns `8`, while the `assert` expects `27`.
+As we could expect, the second test failed. Sure, our function always returns `8`, while the `assert` expects `81`.
 
 ## Improving the implementation
 

1-js/03-code-quality/05-testing-mocha/pow-2.view/test.js

@@ -4,8 +4,8 @@ describe("pow", function() {
     assert.equal(pow(2, 3), 8);
   });
 
-  it("3 raised to power 3 is 27", function() {
-    assert.equal(pow(3, 3), 27);
+  it("3 raised to power 4 is 81", function() {
+    assert.equal(pow(3, 4), 81);
   });
 
 });

1-js/05-data-types/01-primitives-methods/article.md

@@ -7,7 +7,7 @@ Let's look at the key distinctions between primitives and objects.
 A primitive
 
 - Is a value of a primitive type.
-- There are 6 primitive types: `string`, `number`, `boolean`, `symbol`, `null` and `undefined`.
+- There are 7 primitive types: `string`, `number`, `bigint`, `boolean`, `symbol`, `null` and `undefined`.
 
 An object
 

1-js/05-data-types/02-number/article.md

@@ -413,7 +413,7 @@ There are more functions and constants in `Math` object, including trigonometry,
 To write numbers with many zeroes:
 
 - Append `"e"` with the zeroes count to the number. Like: `123e6` is the same as `123` with 6 zeroes `123000000`.
-- A negative number after `"e"` causes the number to be divided by 1 with given zeroes. E.g. `123e-6` means `0.000123` (`123` millionth).
+- A negative number after `"e"` causes the number to be divided by 1 with given zeroes. E.g. `123e-6` means `0.000123` (`123` millionths).
 
 For different numeral systems:
 

1-js/05-data-types/05-array-methods/12-reduce-object/_js.view/solution.js

@@ -0,0 +1,6 @@
+function groupById(array) {
+  return array.reduce((obj, value) => {
+    obj[value.id] = value;
+    return obj;
+  }, {})
+}

1-js/05-data-types/05-array-methods/12-reduce-object/_js.view/test.js

@@ -0,0 +1,20 @@
+describe("groupById", function() {
+
+  it("creates an object grouped by id", function() {
+    let users = [
+      {id: 'john', name: "John Smith", age: 20},
+      {id: 'ann', name: "Ann Smith", age: 24},
+      {id: 'pete', name: "Pete Peterson", age: 31},
+    ];
+
+    assert.deepEqual(groupById(users), {
+      john: {id: 'john', name: "John Smith", age: 20}
+      ann: {id: 'ann', name: "Ann Smith", age: 24},
+      pete: {id: 'pete', name: "Pete Peterson", age: 31},
+    });
+  });
+
+  it("works with an empty array", function() {
+    assert.deepEqual(groupById(users), {});
+  });
+});

1-js/05-data-types/05-array-methods/12-reduce-object/solution.md

@@ -0,0 +1,37 @@
+importance: 4
+
+---
+
+# Create keyed object from array
+
+Let's say we received an array of users in the form `{id:..., name:..., age... }`.
+
+Create a function `groupById(arr)` that creates an object from it, with `id` as the key, and array items as values.
+
+For example:
+
+```js
+let users = [
+  {id: 'john', name: "John Smith", age: 20},
+  {id: 'ann', name: "Ann Smith", age: 24},
+  {id: 'pete', name: "Pete Peterson", age: 31},
+];
+
+let usersById = groupById(users);
+
+/*
+// after the call we have:
+
+usersById = {
+  john: {id: 'john', name: "John Smith", age: 20}
+  ann: {id: 'ann', name: "Ann Smith", age: 24},
+  pete: {id: 'pete', name: "Pete Peterson", age: 31},
+}
+*/
+```
+
+Such function is really handy when working with server data.
+
+In this task we assume that `id` is unique. There may be no two array items with the same `id`.
+
+Please use array `.reduce` method in the solution.

1-js/05-data-types/05-array-methods/12-reduce-object/task.md

@@ -384,7 +384,7 @@ The order became `1, 15, 2`. Incorrect. But why?
 
 **The items are sorted as strings by default.**
 
-Literally, all elements are converted to strings for comparisons. For strings,  lexicographic ordering is applied and indeed `"2" > "15"`.
+Literally, all elements are converted to strings for comparisons. For strings, lexicographic ordering is applied and indeed `"2" > "15"`.
 
 To use our own sorting order, we need to supply a function as the argument of `arr.sort()`.
 
@@ -431,7 +431,6 @@ By the way, if we ever want to know which elements are compared -- nothing preve
 
 The algorithm may compare an element with multiple others in the process, but it tries to make as few comparisons as possible.
 
-
 ````smart header="A comparison function may return any number"
 Actually, a comparison function is only required to return a positive number to say "greater" and a negative number to say "less".
 
@@ -456,6 +455,22 @@ arr.sort( (a, b) => a - b );
 This works exactly the same as the longer version above.
 ````
 
+````smart header="Use `localeCompare` for strings"
+Remember [strings](info:string#correct-comparisons) comparison algorithm? It compares letters by their codes by default.
+
+For many alphabets, it's better to use `str.localeCompare` method to correctly sort letters, such as `Ö`.
+
+For example, let's sort a few countries in German:
+
+```js run
+let countries = ['Österreich', 'Andorra', 'Vietnam'];
+
+alert( countries.sort( (a, b) => a > b ? 1 : -1) ); // Andorra, Vietnam, Österreich (wrong)
+
+alert( countries.sort( (a, b) => a.localeCompare(b) ) ); // Andorra,Österreich,Vietnam (correct!)
+```
+````
+
 ### reverse
 
 The method [arr.reverse](mdn:js/Array/reverse) reverses the order of elements in `arr`.
@@ -530,7 +545,7 @@ The methods [arr.reduce](mdn:js/Array/reduce) and [arr.reduceRight](mdn:js/Array
 The syntax is:
 
 ```js
-let value = arr.reduce(function(previousValue, item, index, array) {
+let value = arr.reduce(function(accumulator, item, index, array) {
   // ...
 }, [initial]);
 ```
@@ -539,14 +554,16 @@ The function is applied to all array elements one after another and "carries on"
 
 Arguments:
 
-- `previousValue` -- is the result of the previous function call, equals `initial` the first time (if `initial` is provided).
+- `accumulator` -- is the result of the previous function call, equals `initial` the first time (if `initial` is provided).
 - `item` -- is the current array item.
 - `index` -- is its position.
 - `array` -- is the array.
 
 As function is applied, the result of the previous function call is passed to the next one as the first argument.
 
-Sounds complicated, but it's not if you think about the first argument as the "accumulator" that stores the combined result of all previous execution. And at the end it becomes the result of `reduce`.
+So, the first argument is essentially the accumulator that stores the combined result of all previous executions. And at the end it becomes the result of `reduce`.
+
+Sounds complicated?
 
 The easiest way to grasp that is by example.
 
@@ -611,7 +628,6 @@ let arr = [];
 arr.reduce((sum, current) => sum + current);
 ```
 
-
 So it's advised to always specify the initial value.
 
 The method [arr.reduceRight](mdn:js/Array/reduceRight) does the same, but goes from right to left.