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Arrays provide a lot of methods.… , one after another: item is the element. index is its index. array is the array itself.… It calls the function for each element of the array and returns the array of results.… item. index – is its position. array – is the array.… On the second run, sum = 1, we add the second array element (2) to it and return.

Typed arrays behave like regular arrays: have indexes and are iterable.… If an Array, or any array-like object is given, it creates a typed array of the same length and copies… These methods allow us to copy typed arrays, mix them, create new arrays from existing ones, and so on… For typed arrays, the constructor dictates what the format is.… Unlike typed arrays, DataView doesn’t create a buffer on its own.

Methods that work with the end of the array: pop Extracts the last element of the array and returns… Methods that work with the beginning of the array: shift Extracts the first element of the array and… exactly the same array.… It is auto-adjusted by array methods. If we shorten length manually, the array is truncated.… treatment for arrays.

For example, to calculate pow(2, 4) the recursive variant does these steps: pow(2, 4) = 2 * pow(2, 3… ) pow(2, 3) = 2 * pow(2, 2) pow(2, 2) = 2 * pow(2, 1) pow(2, 1) = 2 So, the recursion reduces a function… Here’s the context stack when we entered the subcall pow(2, 2): Context: { x: 2, n: 2, at line… n: 1, at line 1 } pow(2, 1) Context: { x: 2, n: 2, at line 5 } pow(2, 2)… pow(2, 2) Context: { x: 2, n: 3, at line 5 } pow(2, 3) The execution of pow(2, 2) is

So when worker.slow(2) is executed, the wrapper gets 2 as an argument and this=worker (it’s the object… The apply accepts only array-like args.… …And for objects that are both iterable and array-like, such as a real array, we can use any of them,… a real array.… It is quite common to take array methods and apply them to arguments.

Promise.all takes an iterable (usually, an array… The new promise resolves when all listed promises are resolved, and the array of their results becomes… For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3… A common trick is to map an array of job data into an array of promises, and then wrap that into Promise.all… But if any of those objects is not a promise, it’s passed to the resulting array “as is”.

If we pass an array of properties to it, only these properties will be encoded.… Fortunately, we can use a function instead of an array as the replacer.… Here space = 2 tells JavaScript to show nested objects on multiple lines, with indentation of 2 spaces… can include other objects and arrays.… JSON supports plain objects, arrays, strings, numbers, booleans, and null.

That has two effects: It allows to get a part of the match as a separate item in the result array.… At index 2: the contents of the second parentheses.… , the corresponding result array item is present and equals undefined.… When the flag g is present, it returns every match as an array with groups.… We can turn it into a real Array using Array.from.

And also, how to pass arrays to such functions as parameters.… The dots literally mean “gather the remaining parameters into an array”.… But the downside is that although arguments is both array-like and iterable, it’s not an array.… Spread syntax.We’ve just seen how to get an array from the list of parameters.… The list of characters is passed to array initializer [...str].

It has 3 modes: If the regexp doesn’t have flag g, then it returns the first match as an array with… If there are no matches, we don’t get an empty array, but null.… There are 3 differences from match: It returns an iterable object with matches instead of an array.… We can make a regular array from it using Array.from.… Every match is returned as an array with capturing groups (the same format as str.match without flag

For instance, when we create an array [1, 2, 3], the default new Array() constructor is used internally… before, Object.prototype has toString as well, but Array.prototype is closer in the chain, so the array… For instance, if we’re making an array-like object, we may want to copy some Array methods to it.… It doesn’t check if the object is indeed an array. Many built-in methods are like that.… __proto__ to Array.prototype, so all Array methods are automatically available in obj.

And it can’t be both version 1 and 2.… from 2 to 3, from 3 to 4 etc.… by step, for every intermediate version (2 to 3, then 3 to 4).… Get the store object using transaction.objectStore(name), at (2).… The price is not an array, so multiEntry flag is not applicable.

Arrays are used for storing ordered collections. But that’s not enough for real life.… isn’t the right way to use a Map Although map[key] also works, e.g. we can set map[key] = 2,… Besides that, Map has a built-in forEach method, similar to Array:… Object.entries: Map from Object.When a Map is created, we can pass an array… Not necessarily an array.

It has 3 working modes: If the regular expression has flag g, it returns an array of all matches:… If there’s no such flag it returns only the first match in the form of an array, with the full match… The array… If there are no matches, we don’t receive an empty array, but instead receive null.… part of the string before the match $' inserts a part of the string after the match $n if n is a 1-2

…But please note: the example above shows 1, then 2, and that’s all. It doesn’t show 3!… It starts the execution and returns the result of the first yield "2+2=?".… For instance, here the yield of "2 + 2 = ?… The error, thrown into the generator at line (2)… The current line of the calling code is the line with generator.throw, labelled as (2).

The method comes in handy when we are iterating over elements (like in an array or something) and trying… The second scripts runs after the browser meets one more <div>, so its length is 2.… It’s like a fixed array of elements.

constructor syntax is: blobParts is an array… From Blob to stream.When we read and write to a blob of more than 2… can easily convert between Blob and low-level binary data types: We can make a Blob from a typed array

The following example will show the message every 2 seconds.… The actual interval between alerts will be shorter than 2 seconds.… Each call remembers the real time from the previous one in the times array.

string, "arraybuffer" – get as ArrayBuffer (for binary data, see chapter ArrayBuffer, binary arrays… An XMLHttpRequest object travels them in the order 0 → 1 → 2

That’s the array of subprotocols, e.g. if we’d like to use SOAP or WAMP:… values: 0 – “CONNECTING”: the connection has not yet been established, 1 – “OPEN”: communicating, 2

Of course, Arrays are iterable.… Array-likes are objects that have indexes and length, so they look like arrays.… But an iterable may not be array-like. And vice versa an array-like may not be iterable.… a “real” Array from it.… use array methods on it.

They are supported for: Map Set Array Plain objects also support similar methods, but the syntax is… Object.values(obj) – returns an array of values.… Object.entries(obj) – returns an array of [key, value] pairs.… Use array methods on that array, e.g. map, to transform these key/value pairs.… Use Object.fromEntries(array) on the resulting array to turn it back into an object.

Built-in classes like Array, Map and others are extendable also.… of results using exactly arr.constructor, not basic Array.… If we’d like built-in methods like map or filter to return regular arrays, we can return Array in Symbol.species… For instance, Array extends Object.… But Array.

The two most used data structures in JavaScript are Object and Array.… Arrays allow us to gather data items into an ordered list.… Array destructuring.Here’s an example of how an array is destructured into variables:… However, the array itself is not modified.… of the remaining array elements.

Otherwise, if the first digit is 2, then the next must be [0-3].… (no other first digit is allowed) We can write both variants in a regexp using alternation: [01]\d|2[… If we glue hours and minutes together, we get the pattern: [01]\d|2[0-3]:[0-5]\d.… The alternation | now happens to be between [01]\d and 2[0-3]:[0-5]\d.… Let’s correct that by enclosing “hours” into parentheses: ([01]\d|2[0-3]):[0-5]\d.

There may be 2, 3, 4 or more.… In the demo above they are labeled: 1, 2, 3. Build segments between control points 1 → 2 → 3.… 2 → 3, 3 → 4.… There will be N-2 segments. Repeat step 2 until there is only one point.… 0 + 2(1−t)t * 0.5 + t2 * 1 = (1-t)t + t2 = t y = (1−t)2 * 0 + 2(1−t)t * 1 + t2 * 0 = 2(1-t)t = –2t2 +

object with two properties: done – true when the reading is complete, otherwise false. value – a typed array… We gather response chunks in the array chunks.… At the end, we have chunks – an array of Uint8Array byte chunks.… there’s some code to do that: We create chunksAll = new Uint8Array(receivedLength) – a same-typed array… It’s a byte array though, not a string. To create a string, we need to interpret these bytes.

operand is 2.… The 2 gets concatenated to '1', so it’s like '1' + 2 = "12" and "12" + 2 = "… For example, write (1 + 2) * 2. There are many operators in JavaScript.… So, the alert shows 2.… It’s like (a = 1 + 2), 3 + 4.

DOM collections.As we can see, childNodes looks like an array.… But actually it’s not an array, but rather a collection – a special array-like iterable object.… Array methods won’t work, because it’s not an array:… The second is tolerable, because we can use Array.from to create a “real” array from the collection,… if we want array methods:

Surrogate pairs.All frequently used characters have 2-byte codes (4 hex digits).… Initially, JavaScript was based on UTF-16 encoding that only allowed 2 bytes per character.… But 2 bytes only allow 65536 combinations and that’s not enough for every possible symbol of Unicode.… So rare symbols that require more than 2 bytes are encoded with a pair of 2-byte characters called “a… As a side effect, the length of such symbols is 2:

…And with built-in classes like Array… That’s because Array prototypically inherits from Object.… boolean, it will be [object Boolean] For null: [object Null] For undefined: [object Undefined] For arrays… : [object Array] …etc (customizable).

Most characters are encoded with 2 bytes, but that allows to represent at most 65536 characters.… are the Unicode values of some characters: Character Unicode Bytes count in Unicode a 0x0061 2… ≈ 0x2248 2 𝒳 0x1d4b3 4 𝒴 0x1d4b4 4 😄 0x1f604 4 So characters like a and ≈ occupy 2 bytes… The point is that length treats 4 bytes as two 2-byte characters.… By default, regular expressions also treat 4-byte “long characters” as a pair of 2-byte ones.

second (*), Then the .then handler is called (**), which in turn creates a new promise (resolved with 2… As the result is passed along the chain of handlers, we can see a sequence of alert calls: 1 → 2 → 4.… That handler is in the line (**), it shows 2 and does the same thing.… So the output is the same as in the previous example: 1 → 2 → 4, but now with 1 second delay between… In the example above resolve(2) is called after 1 second (**).

base=2 is mostly for debugging bitwise operations, digits can be 0 or 1.… several built-in functions for rounding: Math.floor Rounds down: 3.1 becomes 3, and -1.1 becomes -2.… -1 -1 -1 -1.5 -2 -1 -1 -1 -1.6 -2 -1 -2 -1 These functions cover all of the possible ways to deal… For instance, we have 1.2345 and want to round it to 2 digits, getting only 1.23.… It ensures that it has 2 digits after the decimal point.

For example, the pattern \b\d\d\b looks for standalone 2-digit numbers.… In other words, it looks for 2-digit numbers that are surrounded by characters different from \w, such

The multiplication obj * 2… first converts the object to primitive (that’s a string "2").… Then "2" * 2 becomes 2 * 2 (the string is converted to number).

Usually, properties of an object or elements of an array… For instance, if we put an object into an array, then while the array is alive, the object will be alive

similar to Blob: fileParts – is an array… Please note: The input may select multiple files, so input.files is an array-like

We’ll make a numeric array that returns 0 for nonexistent values.… Usually when one tries to get a non-existing array item, they get undefined, but we’ll wrap a regular… Validation with “set” trap.Let’s say we want an array exclusively for numbers.… Array has no internal slots A notable exception: built-in Array doesn’t use internal slots… So there’s no such problem when proxying an array.

For compatibility, 2 digits are also accepted and considered 19xx, e.g. 98 is the same as 1998 here,… It returns 2-digit year sometimes. Please never use it. There is getFullYear() for the year.… Let’s say we need to increase the date “28 Feb 2016” by 2 days.… It may be “2 Mar” or “1 Mar” in case of a leap-year. We don’t need to think about it.… Just add 2 days.

"Very plain" objects.As we know, objects can be used as associative arrays to store key/value… Now, if we intend to use an object as an associative array and be free of such problems, we can do it… …But that’s usually fine for associative arrays

Solution 2: bind.Functions provide a built-in method bind that allows to fix this.… The call to mul.bind(null, 2)… creates a new function double that passes calls to mul, fixing null as the context and 2 as the first

Indents.There are two types of indents: Horizontal indents: 2 or 4 spaces.… A horizontal indentation is made using either 2 or 4 spaces or the horizontal tab symbol (key Tab).… Option 1: Option 2:

state event.button Left button (primary) 0 Middle button (auxiliary) 1 Right button (secondary) 2… (forward) 4 Most mouse devices only have the left and right buttons, so possible values are 0 or 2.… non-standard way of getting a button, with possible values: event.which == 1 – left button, event.which == 2

As we can see, there are 2

Summary.Objects are associative arrays with several special features.… There are many other kinds of objects in JavaScript: Array to store ordered data collections, Date to… Sometimes people say something like “Array type” or “Date type”, but formally they are not types of their

That includes JavaScript built-ins, such as Array and environment-specific values, such as window.innerHeight

Assignments There is a simple assignment: a = b and combined ones like a *= 2.… Parameters can have default values: function sum(a = 1, b = 2) {...}.

For 2 the second alert runs.

Similar to that, \2 would mean the contents of the second group, \3 – the 3rd group, and so on.