Merge branch 'master' of github.com:javascript-tutorial/en.javascript.info into sync-0599d07b

Author: iliakan

1-js/01-getting-started/1-intro/article.md

@@ -1,6 +1,6 @@
 # An Introduction to JavaScript
 
-Let's see what's so special about JavaScript, what we can achieve with it, and which other technologies play well with it.
+Let's see what's so special about JavaScript, what we can achieve with it, and what other technologies play well with it.
 
 ## What is JavaScript?
 
@@ -68,7 +68,7 @@ Examples of such restrictions include:
     Modern browsers allow it to work with files, but the access is limited and only provided if the user does certain actions, like "dropping" a file into a browser window or selecting it via an `<input>` tag.
 
     There are ways to interact with camera/microphone and other devices, but they require a user's explicit permission. So a JavaScript-enabled page may not sneakily enable a web-camera, observe the surroundings and send the information to the [NSA](https://en.wikipedia.org/wiki/National_Security_Agency).
-- Different tabs/windows generally do not know about each other. Sometimes they do, for example when one window uses JavaScript to open the other one. But even in this case, JavaScript from one page may not access the other if they come from different sites (from a different domain, protocol or port).
+- Different tabs/windows generally do not know about each other. Sometimes they do; for example when one window uses JavaScript to open the other one. But even in this case, JavaScript from one page may not access the other if they come from different sites (from a different domain, protocol or port).
 
     This is called the "Same Origin Policy". To work around that, *both pages* must agree for data exchange and contain a special JavaScript code that handles it. We'll cover that in the tutorial.
 
@@ -116,6 +116,6 @@ There are more. Of course, even if we use one of transpiled languages, we should
 
 ## Summary
 
-- JavaScript was initially created as a browser-only language, but is now used in many other environments as well.
-- Today, JavaScript has a unique position as the most widely-adopted browser language with full integration with HTML/CSS.
+- JavaScript was initially created as a browser-only language, but it is now used in many other environments as well.
+- Today, JavaScript has a unique position as the most widely-adopted browser language with full integration in HTML/CSS.
 - There are many languages that get "transpiled" to JavaScript and provide certain features. It is recommended to take a look at them, at least briefly, after mastering JavaScript.

1-js/02-first-steps/18-javascript-specials/article.md

@@ -144,7 +144,7 @@ Assignments
 : There is a simple assignment: `a = b` and combined ones like `a *= 2`.
 
 Bitwise
-: Bitwise operators work with 32-bit integers at the lowest, bit-level: see the [docs](mdn:/JavaScript/Reference/Operators/Bitwise_Operators) when they are needed.
+: Bitwise operators work with 32-bit integers at the lowest, bit-level: see the [docs](mdn:/JavaScript/Guide/Expressions_and_Operators#Bitwise) when they are needed.
 
 Conditional
 : The only operator with three parameters: `cond ? resultA : resultB`. If `cond` is truthy, returns `resultA`, otherwise `resultB`.

1-js/04-object-basics/01-object/8-multiply-numeric/task.md

@@ -2,9 +2,9 @@ importance: 3
 
 ---
 
-# Multiply numeric properties by 2
+# Multiply numeric property values by 2
 
-Create a function `multiplyNumeric(obj)` that multiplies all numeric properties of `obj` by `2`.
+Create a function `multiplyNumeric(obj)` that multiplies all numeric property values of `obj` by `2`.
 
 For instance:
 

1-js/04-object-basics/02-object-copy/article.md

@@ -1,10 +1,12 @@
-# Object copying, references
+# Object references and copying
 
-One of the fundamental differences of objects vs primitives is that they are stored and copied "by reference".
+One of the fundamental differences of objects versus primitives is that objects are stored and copied "by reference", as opposed to primitive values: strings, numbers, booleans, etc -- that are always copied "as a whole value".
 
-Primitive values: strings, numbers, booleans -- are assigned/copied "as a whole value".
+That's easy to understand if we look a bit "under a cover" of what happens when we copy a value.
 
-For instance:
+Let's start with a primitive, such as a string.
+
+Here we put a copy of `message` into `phrase`:
 
 ```js
 let message = "Hello!";
@@ -15,21 +17,31 @@ As a result we have two independent variables, each one is storing the string `"
 
 ![](variable-copy-value.svg)
 
+Quite an obvious result, right?
+
 Objects are not like that.
 
-**A variable stores not the object itself, but its "address in memory", in other words "a reference" to it.**
+**A variable assigned to an object stores not the object itself, but its "address in memory", in other words "a reference" to it.**
 
-Here's the picture for the object:
+Let's look at an example of such variable:
 
 ```js
 let user = {
   name: "John"
 };
 ```
 
+And here's how it's actually stored in memory:
+
 ![](variable-contains-reference.svg)
 
-Here, the object is stored somewhere in memory. And the variable `user` has a "reference" to it.
+The object is stored somewhere in memory (at the right of the picture), while the `user` variable (at the left) has a "reference" to it.
+
+We may think of an object variable, such as `user`, as of a sheet of paper with the address.
+
+When we perform actions with the object, e.g. take a property `user.name`, JavaScript engine looks into that address and performs the operation on the actual object.
+
+Now here's why it's important.
 
 **When an object variable is copied -- the reference is copied, the object is not duplicated.**
 
@@ -45,6 +57,8 @@ Now we have two variables, each one with the reference to the same object:
 
 ![](variable-copy-reference.svg)
 
+As you can see, there's still one object, now with two variables that reference it.
+
 We can use any variable to access the object and modify its contents:
 
 ```js run
@@ -59,15 +73,14 @@ admin.name = 'Pete'; // changed by the "admin" reference
 alert(*!*user.name*/!*); // 'Pete', changes are seen from the "user" reference
 ```
 
-The example above demonstrates that there is only one object. As if we had a cabinet with two keys and used one of them (`admin`) to get into it. Then, if we later use another key (`user`) we can see changes.
 
-## Comparison by reference
+It's just as if we had a cabinet with two keys and used one of them (`admin`) to get into it. Then, if we later use another key (`user`) we can see changes.
 
-The equality `==` and strict equality `===` operators for objects work exactly the same.
+## Comparison by reference
 
-**Two objects are equal only if they are the same object.**
+Two objects are equal only if they are the same object.
 
-Here two variables reference the same object, thus they are equal:
+For instance, here `a` and `b` reference the same object, thus they are equal:
 
 ```js run
 let a = {};
@@ -77,7 +90,7 @@ alert( a == b ); // true, both variables reference the same object
 alert( a === b ); // true
 ```
 
-And here two independent objects are not equal, even though both are empty:
+And here two independent objects are not equal, even though they look alike (both are empty):
 
 ```js run
 let a = {};
@@ -86,7 +99,7 @@ let b = {}; // two independent objects
 alert( a == b ); // false
 ```
 
-For comparisons like `obj1 > obj2` or for a comparison against a primitive `obj == 5`, objects are converted to primitives. We'll study how object conversions work very soon, but to tell the truth, such comparisons occur very rarely, usually as a result of a coding mistake.
+For comparisons like `obj1 > obj2` or for a comparison against a primitive `obj == 5`, objects are converted to primitives. We'll study how object conversions work very soon, but to tell the truth, such comparisons are needed very rarely, usually they appear as a result of a programming mistake.
 
 ## Cloning and merging, Object.assign
 

1-js/04-object-basics/04-object-methods/article.md

@@ -32,11 +32,11 @@ user.sayHi = function() {
 user.sayHi(); // Hello!
 ```
 
-Here we've just used a Function Expression to create the function and assign it to the property `user.sayHi` of the object.
+Here we've just used a Function Expression to create a function and assign it to the property `user.sayHi` of the object.
 
-Then we can call it. The user can now speak!
+Then we can call it as `user.sayHi()`. The user can now speak!
 
-A function that is the property of an object is called its *method*.
+A function that is a property of an object is called its *method*.
 
 So, here we've got a method `sayHi` of the object `user`.
 
@@ -160,14 +160,16 @@ let user = {
 let admin = user;
 user = null; // overwrite to make things obvious
 
-admin.sayHi(); // Whoops! inside sayHi(), the old name is used! error!
+*!*
+admin.sayHi(); // TypeError: Cannot read property 'name' of null
+*/!*
 ```
 
 If we used `this.name` instead of `user.name` inside the `alert`, then the code would work.
 
 ## "this" is not bound
 
-In JavaScript, keyword `this` behaves unlike most other programming languages. It can be used in any function.
+In JavaScript, keyword `this` behaves unlike most other programming languages. It can be used in any function, even if it's not a method of an object.
 
 There's no syntax error in the following example:
 

1-js/04-object-basics/07-optional-chaining/article.md

@@ -3,21 +3,25 @@
 
 [recent browser="new"]
 
-The optional chaining `?.` is an error-proof way to access nested object properties, even if an intermediate property doesn't exist.
+The optional chaining `?.` is a safe way to access nested object properties, even if an intermediate property doesn't exist.
 
-## The problem
+## The "non-existing property" problem
 
 If you've just started to read the tutorial and learn JavaScript, maybe the problem hasn't touched you yet, but it's quite common.
 
-For example, some of our users have addresses, but few did not provide them. Then we can't safely read `user.address.street`:
+As an example, let's consider objects for user data. Most of our users have addresses in `user.address` property, with the street `user.address.street`, but some did not provide them. 
+
+In such case, when we attempt to get `user.address.street`, we'll get an error:
 
 ```js run
-let user = {}; // the user happens to be without address
+let user = {}; // the user without "address" property
 
 alert(user.address.street); // Error!
 ```
 
-Or, in the web development, we'd like to get an information about an element on the page, but it may not exist:
+That's the expected result, JavaScript works like this, but many practical cases we'd prefer to get `undefined` instead of an error (meaning "no street").
+
+...And another example. In the web development, we may need to get an information about an element on the page, that sometimes doesn't exist:
 
 ```js run
 // Error if the result of querySelector(...) is null
@@ -34,7 +38,7 @@ let user = {}; // user has no address
 alert( user && user.address && user.address.street ); // undefined (no error)
 ```
 
-AND'ing the whole path to the property ensures that all components exist, but is cumbersome to write.
+AND'ing the whole path to the property ensures that all components exist (if not, the evaluation stops), but is cumbersome to write.
 
 ## Optional chaining
 
@@ -70,7 +74,7 @@ We should use `?.` only where it's ok that something doesn't exist.
 
 For example, if according to our coding logic `user` object must be there, but `address` is optional, then `user.address?.street` would be better.
 
-So, if `user` happens to be undefined due to a mistake, we'll know about it and fix it. Otherwise, coding errors can be silenced where not appropriate, and become more difficult to debug.
+So, if `user` happens to be undefined due to a mistake, we'll see a programming error about it and fix it. Otherwise, coding errors can be silenced where not appropriate, and become more difficult to debug.
 ```
 
 ````warn header="The variable before `?.` must be declared"
@@ -80,25 +84,27 @@ If there's no variable `user` at all, then `user?.anything` triggers an error:
 // ReferenceError: user is not defined
 user?.address;
 ```
-There must be `let/const/var user`. The optional chaining works only for declared variables.
+There must be a declaration (e.g. `let/const/var user`). The optional chaining works only for declared variables.
 ````
 
 ## Short-circuiting
 
 As it was said before, the `?.` immediately stops ("short-circuits") the evaluation if the left part doesn't exist.
 
-So, if there are any further function calls or side effects, they don't occur:
+So, if there are any further function calls or side effects, they don't occur.
+
+For instance:
 
 ```js run
 let user = null;
 let x = 0;
 
-user?.sayHi(x++); // nothing happens
+user?.sayHi(x++); // no "sayHi", so the execution doesn't reach x++
 
 alert(x); // 0, value not incremented
 ```
 
-## Other cases: ?.(), ?.[]
+## Other variants: ?.(), ?.[]
 
 The optional chaining `?.` is not an operator, but a special syntax construct, that also works with functions and square brackets.
 
@@ -121,9 +127,9 @@ user2.admin?.();
 */!*
 ```
 
-Here, in both lines we first use the dot `.` to get `admin` property, because the user object must exist, so it's safe read from it.
+Here, in both lines we first use the dot (`user1.admin`) to get `admin` property, because the user object must exist, so it's safe read from it.
 
-Then `?.()` checks the left part: if the admin function exists, then it runs (for `user1`). Otherwise (for `user2`) the evaluation stops without errors.
+Then `?.()` checks the left part: if the admin function exists, then it runs (that's so for `user1`). Otherwise (for `user2`) the evaluation stops without errors.
 
 The `?.[]` syntax also works, if we'd like to use brackets `[]` to access properties instead of dot `.`. Similar to previous cases, it allows to safely read a property from an object that may not exist.
 
@@ -148,28 +154,30 @@ Also we can use `?.` with `delete`:
 delete user?.name; // delete user.name if user exists
 ```
 
-```warn header="We can use `?.` for safe reading and deleting, but not writing"
-The optional chaining `?.` has no use at the left side of an assignment:
+````warn header="We can use `?.` for safe reading and deleting, but not writing"
+The optional chaining `?.` has no use at the left side of an assignment.
 
+For example:
 ```js run
-// the idea of the code below is to write user.name, if user exists
+let user = null;
 
 user?.name = "John"; // Error, doesn't work
 // because it evaluates to undefined = "John"
 ```
 
+It's just not that smart.
+````
+
 ## Summary
 
-The `?.` syntax has three forms:
+The optional chaining `?.` syntax has three forms:
 
 1. `obj?.prop` -- returns `obj.prop` if `obj` exists, otherwise `undefined`.
 2. `obj?.[prop]` -- returns `obj[prop]` if `obj` exists, otherwise `undefined`.
-3. `obj?.method()` -- calls `obj.method()` if `obj` exists, otherwise returns `undefined`.
+3. `obj.method?.()` -- calls `obj.method()` if `obj.method` exists, otherwise returns `undefined`.
 
 As we can see, all of them are straightforward and simple to use. The `?.` checks the left part for `null/undefined` and allows the evaluation to proceed if it's not so.
 
 A chain of `?.` allows to safely access nested properties.
 
-Still, we should apply `?.` carefully, only where it's ok that the left part doesn't to exist.
-
-So that it won't hide programming errors from us, if they occur.
+Still, we should apply `?.` carefully, only where it's acceptable that the left part doesn't to exist. So that it won't hide programming errors from us, if they occur.

1-js/05-data-types/03-string/article.md

@@ -110,7 +110,7 @@ alert( 'I*!*\'*/!*m the Walrus!' ); // *!*I'm*/!* the Walrus!
 
 As you can see, we have to prepend the inner quote by the backslash `\'`, because otherwise it would indicate the string end.
 
-Of course, only to the quotes that are the same as the enclosing ones need to be escaped. So, as a more elegant solution, we could switch to double quotes or backticks instead:
+Of course, only the quotes that are the same as the enclosing ones need to be escaped. So, as a more elegant solution, we could switch to double quotes or backticks instead:
 
 ```js run
 alert( `I'm the Walrus!` ); // I'm the Walrus!
@@ -312,7 +312,7 @@ if (str.indexOf("Widget") != -1) {
 
 #### The bitwise NOT trick
 
-One of the old tricks used here is the [bitwise NOT](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators#Bitwise_NOT) `~` operator. It converts the number to a 32-bit integer (removes the decimal part if exists) and then reverses all bits in its binary representation.
+One of the old tricks used here is the [bitwise NOT](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_NOT) `~` operator. It converts the number to a 32-bit integer (removes the decimal part if exists) and then reverses all bits in its binary representation.
 
 In practice, that means a simple thing: for 32-bit integers `~n` equals `-(n+1)`.
 
@@ -345,7 +345,7 @@ It is usually not recommended to use language features in a non-obvious way, but
 
 Just remember: `if (~str.indexOf(...))` reads as "if found".
 
-To be precise though, as big numbers are truncated to 32 bits by `~` operator, there exist other numbers that give `0`, the smallest is `~4294967295=0`. That makes such check is correct only if a string is not that long.
+To be precise though, as big numbers are truncated to 32 bits by `~` operator, there exist other numbers that give `0`, the smallest is `~4294967295=0`. That makes such check correct only if a string is not that long.
 
 Right now we can see this trick only in the old code, as modern JavaScript provides `.includes` method (see below).
 

1-js/05-data-types/05-array-methods/7-map-objects/solution.md

@@ -25,7 +25,7 @@ alert( usersMapped[0].id ); // 1
 alert( usersMapped[0].fullName ); // John Smith
 ```
 
-Please note that in for the arrow functions we need to use additional brackets. 
+Please note that in the arrow functions we need to use additional brackets. 
 
 We can't write like this:
 ```js

1-js/05-data-types/05-array-methods/8-sort-objects/solution.md

@@ -1,6 +1,6 @@
 ```js run no-beautify
 function sortByAge(arr) {
-  arr.sort((a, b) => a.age > b.age ? 1 : -1);
+  arr.sort((a, b) => a.age - b.age);
 }
 
 let john = { name: "John", age: 25 };