Šeit ir piemēri visam jaunajam ECMAScript 2016, 2017 un 2018

Ir grūti izsekot JavaScript jaunumiem (ECMAScript). Un vēl grūtāk ir atrast noderīgus kodu piemērus.

Tāpēc šajā rakstā es aplūkošu visas 18 funkcijas, kas uzskaitītas TC39 gatavajos priekšlikumos, kas tika pievienotas ES2016, ES2017 un ES2018 (galīgais projekts), un parādīšu tos ar noderīgiem piemēriem.

Šis ir diezgan garš ieraksts, taču to vajadzētu viegli izlasīt. Padomājiet par to kā par “ Netflix pārmērīgu lasīšanu”. Līdz tam es apsolu, ka jums būs daudz zināšanu par visām šīm funkcijām.

Labi, ejam pa šiem pāri.

1. Array.prototype.includes

includesir vienkārša masīva instances metode un palīdz viegli atrast, vai masīvā ir kāds vienums (ieskaitot NaNatšķirībā no tā indexOf).

Trivia: JavaScript spec cilvēki gribēja to nosaukt contains, bet acīmredzot to jau izmantoja Mootools, tāpēc viņi to izmantoja includes.

2. Paaugstināšana infix operator

Matemātikas operācijām, piemēram, saskaitīšanai un atņemšanai, ir attiecīgi infiksu operatori +un -. Līdzīgi kā tie, **infiksu operators parasti tiek izmantots eksponenta darbībai. ECMAScript 2016 ** tika ieviests nevis Math.pow.

1. Object.values ​​()

Object.values()ir jauna funkcija, kas ir līdzīga, Object.keys()bet atgriež visas objekta īpašību vērtības, izņemot visas vērtības prototipiskajā ķēdē.

2. Object.entries ()

Object.entries()ir saistīts ar Object.keys, bet tā vietā, lai atgrieztu tikai atslēgas, tas atgriež gan atslēgas, gan vērtības masīva veidā. Tas padara ļoti vienkāršu tādu darbību veikšanu kā objektu izmantošana lokos vai objektu pārveidošana Maps.

1. piemērs:

2. piemērs:

3. Stīgu polsterējums

String - String.prototype.padStart un String.prototype.padEnd- tika pievienotas divas instances metodes, kas ļauj pievienot vai sagatavot tukšu virkni vai kādu citu virkni sākotnējās virknes sākumam vai beigām.

'someString'.padStart(numberOfCharcters [,stringForPadding]); '5'.padStart(10) // ' 5' '5'.padStart(10, '=*') //'=*=*=*=*=5' '5'.padEnd(10) // '5 ' '5'.padEnd(10, '=*') //'5=*=*=*=*='
Tas ir noderīgi, ja mēs vēlamies saskaņot lietas tādos scenārijos kā glīts drukas displejs vai termināla druka.

Sākt piemēru:

Zemāk esošajā piemērā mums ir dažāda garuma numuru saraksts. Mēs vēlamies ievadīt “0”, lai displeja nolūkos visiem vienumiem būtu vienāds 10 ciparu garums. Mēs varam izmantot, padStart(10, '0')lai to viegli sasniegtu.

3.2. Paliktnis. Beigu piemērs:

padEnd tiešām noderēs, kad drukājam vairākus dažāda garuma priekšmetus un vēlamies tos pareizi izlīdzināt pa labi.

Turpmāk minētais piemērs ir labs reāli piemērs tam, kā padEnd, padStartun Object.entriesvisi nāk kopā, lai radītu skaistu produkciju.

const cars = { '?BMW': '10', '?Tesla': '5', '?Lamborghini': '0' } Object.entries(cars).map(([name, count]) => { //padEnd appends ' -' until the name becomes 20 characters //padStart prepends '0' until the count becomes 3 characters. console.log(`${name.padEnd(20, ' -')} Count: ${count.padStart(3, '0')}`) }); //Prints.. // ?BMW - - - - - - - Count: 010 // ?Tesla - - - - - - Count: 005 // ?Lamborghini - - - Count: 000

3.3 ⚠️ padStart un padEnd Emojis un citās divbaitu rakstzīmēs

Emocijas un citas divbaitu rakstzīmes tiek attēlotas, izmantojot vairākus unikoda baitus. Tātad padStart un padEnd var nedarboties, kā paredzēts! ⚠️

Piemēram: Pieņemsim, ka mēs mēģinām palikt virkni, heartlai sasniegtu 10rakstzīmes ar emocijzīmi ❤️. Rezultāts izskatīsies šādi:

//Notice that instead of 5 hearts, there are only 2 hearts and 1 heart that looks odd! 'heart'.padStart(10, "❤️"); // prints.. '❤️❤️❤heart'

Tas ir tāpēc, ka ❤️ ir 2 koda punkti ( '\u2764\uFE0F')! Pats vārds heartsastāv no 5 rakstzīmēm, tāpēc mums ir palikuši tikai 5 simboli, kas jāpieliek. Tātad notiek tas, ka JS izmanto divas sirdis, '\u2764\uFE0F'un tas rada ❤️❤️. Pēdējam vienkārši tiek izmantots pirmais sirds baits, \u2764kas rada ❤

Tātad mēs nonākam pie: ❤️❤️❤heart

PS: šo saiti varat izmantot, lai pārbaudītu unikoda pārrēķinus.

4. Object.getOwnPropertyDescriptors

Šī metode atgriež visu informāciju (ieskaitot getter getun setter setmetodes) par visām konkrētā objekta īpašībām. Galvenā motivācija to pievienot ir ļaut sekli kopēt / klonēt objektu citā objektākas kopē arī getter un setter funkcijas atšķirībā no Object.assign.

Object.assign sekla kopē visu informāciju, izņemot sākotnējā avota objekta getter un setter funkcijas.

Piemērs ir zemāk redzamajā atšķirība starp Object.assign, un Object.getOwnPropertyDescriptorskopā ar Object.definePropertieskopēt oriģinālo objektu Caruz jaunu objektu ElectricCar. Jūs redzēsiet, ka, izmantojot Object.getOwnPropertyDescriptors, discountfunkcijas getter un setter tiek kopētas arī mērķa objektā.

PIRMS…

PĒC…

var Car = { name: 'BMW', price: 1000000, set discount(x) { this.d = x; }, get discount() { return this.d; }, }; //Print details of Car object's 'discount' property console.log(Object.getOwnPropertyDescriptor(Car, 'discount')); //prints.. // { // get: [Function: get], // set: [Function: set], // enumerable: true, // configurable: true // } //Copy Car's properties to ElectricCar using Object.assign const ElectricCar = Object.assign({}, Car); //Print details of ElectricCar object's 'discount' property console.log(Object.getOwnPropertyDescriptor(ElectricCar, 'discount')); //prints.. // { // value: undefined, // writable: true, // enumerable: true, // configurable: true // } //⚠️Notice that getters and setters are missing in ElectricCar object for 'discount' property !?? //Copy Car's properties to ElectricCar2 using Object.defineProperties //and extract Car's properties using Object.getOwnPropertyDescriptors const ElectricCar2 = Object.defineProperties({}, Object.getOwnPropertyDescriptors(Car)); //Print details of ElectricCar2 object's 'discount' property console.log(Object.getOwnPropertyDescriptor(ElectricCar2, 'discount')); //prints.. // { get: [Function: get], ?????? // set: [Function: set], ?????? // enumerable: true, // configurable: true // } // Notice that getters and setters are present in the ElectricCar2 object for 'discount' property!

5. Add trailing commas in the function parameters

Šis ir neliels atjauninājums, kas ļauj mums pēc pēdējās funkcijas parametra būt ar komatiem. Kāpēc? Lai palīdzētu ar tādiem rīkiem kā git vainot, lai nodrošinātu, ka tiek vainoti tikai jaunie izstrādātāji.

Zemāk sniegtajā piemērā parādīta problēma un risinājums.

Piezīme. Funkcijas var izsaukt arī ar komatiem!

6. Async / Await

This, by far, is the most important and most useful feature if you ask me. Async functions allows us to not deal with callback hell and make the entire code look simple.

The async keyword tells the JavaScript compiler to treat the function differently. The compiler pauses whenever it reaches the await keyword within that function. It assumes that the expression after await returns a promise and waits until the promise is resolved or rejected before moving further.

In the example below, the getAmount function is calling two asynchronous functions getUser and getBankBalance . We can do this in promise, but using async await is more elegant and simple.

6.1 Async functions themselves return a Promise.

If you are waiting for the result from an async function, you need to use Promise’s then syntax to capture its result.

In the following example, we want to log the result using console.log but not within the doubleAndAdd. So we want to wait and use then syntax to pass the result to console.log .

6.2 Calling async/await in parallel

In the previous example we are calling await twice, but each time we are waiting for one second (total 2 seconds). Instead we can parallelize it since a and b are not dependent on each other using Promise.all.

6.3 Error handling async/await functions

There are various ways to handle errors when using async await.

Option 1 — Use try catch within the function

//Option 1 - Use try catch within the function async function doubleAndAdd(a, b) { try { a = await doubleAfter1Sec(a); b = await doubleAfter1Sec(b); } catch (e) { return NaN; //return something } return a + b; } //?Usage: doubleAndAdd('one', 2).then(console.log); // NaN doubleAndAdd(1, 2).then(console.log); // 6 function doubleAfter1Sec(param) { return new Promise((resolve, reject) => { setTimeout(function() { let val = param * 2; isNaN(val) ? reject(NaN) : resolve(val); }, 1000); }); }

Option 2— Catch every await expression

Tā kā katra awaitizteiksme atgriež solījumu, katrā rindiņā varat pieļaut kļūdas, kā parādīts zemāk.

//Option 2 - *Catch* errors on every await line //as each await expression is a Promise in itself async function doubleAndAdd(a, b) { a = await doubleAfter1Sec(a).catch(e => console.log('"a" is NaN')); // ? b = await doubleAfter1Sec(b).catch(e => console.log('"b" is NaN')); // ? if (!a || !b) { return NaN; } return a + b; } //?Usage: doubleAndAdd('one', 2).then(console.log); // NaN and logs: "a" is NaN doubleAndAdd(1, 2).then(console.log); // 6 function doubleAfter1Sec(param) { return new Promise((resolve, reject) => { setTimeout(function() { let val = param * 2; isNaN(val) ? reject(NaN) : resolve(val); }, 1000); }); }

3. iespēja - noķert visu funkciju asinhronizācija

//Option 3 - Dont do anything but handle outside the function //since async / await returns a promise, we can catch the whole function's error async function doubleAndAdd(a, b) { a = await doubleAfter1Sec(a); b = await doubleAfter1Sec(b); return a + b; } //?Usage: doubleAndAdd('one', 2) .then(console.log) .catch(console.log); // ??? { setTimeout(function() { let val = param * 2; isNaN(val) ? reject(NaN) : resolve(val); }, 1000); }); }
ECMAScript pašlaik ir galīgajā projektā, un tā būs pieejama 2018. gada jūnijā vai jūlijā. Visas tālāk aprakstītās funkcijas ir 4. posmā un būs daļa no ECMAScript 2018.

1. Kopīga atmiņa un atomi

Šī ir milzīga, diezgan uzlabota funkcija, un tā ir JS dzinēju galvenais uzlabojums.

Galvenā ideja ir ieviest sava veida multi-threading funkciju JavaScript, lai JS izstrādātāji nākotnē varētu rakstīt augstas veiktspējas, vienlaicīgas programmas, ļaujot pašiem pārvaldīt atmiņu, nevis ļaujot JS motoram pārvaldīt atmiņu.

This is done by a new type of a global object called SharedArrayBuffer that essentially stores data in a sharedmemory space. So this data can be shared between the main JS thread and web-worker threads.

Until now, if we want to share data between the main JS thread and web-workers, we had to copy the data and send it to the other thread using postMessage . Not anymore!

You simply use SharedArrayBuffer and the data is instantly accessible by both the main thread and multiple web-worker threads.

Bet atmiņas koplietošana starp pavedieniem var izraisīt sacensību apstākļus. Lai izvairītos no sacensību apstākļiem, tiek parādīts globālais objekts “ Atomics ”. Atomics nodrošina dažādas metodes, kā bloķēt koplietojamo atmiņu, kad pavediens izmanto savus datus. Tas arī nodrošina metodes, kā droši atjaunināt šādus datus šajā kopīgajā atmiņā.

Ieteikums ir izmantot šo funkciju, izmantojot kādu bibliotēku, taču šobrīd šai funkcijai nav izveidotas bibliotēkas.

Ja jūs interesē, es iesaku izlasīt:

  1. No darba ņēmējiem līdz kopīgam atmiņai y - lucasfcosta
  2. Karikatūras ievads vietnē SharedArrayBuffers - Lin Clark
  3. Kopīga atmiņa un atomi - doktors Aksels Raušmajers

2. Atzīmēts veidnes burtiskais ierobežojums noņemts

First, we need to clarify what a “Tagged Template literal” is so we can understand this feature better.

In ES2015+, there is a feature called a tagged template literal that allows developers to customize how strings are interpolated. For example, in the standard way strings are interpolated like below…

In the tagged literal, you can write a function to receive the hardcoded parts of the string literal, for example [ ‘Hello ‘, ‘!’ ] , and the replacement variables, for example,[ 'Raja'] , as parameters into a custom function (for example greet ), and return whatever you want from that custom function.

The below example shows that our custom “Tag” function greet appends time of the day like “Good Morning!” “Good afternoon,” and so on depending on the time of the day to the string literal and returns a custom string.

//A "Tag" function returns a custom string literal. //In this example, greet calls timeGreet() to append Good //Morning/Afternoon/Evening depending on the time of the day. function greet(hardCodedPartsArray, ...replacementPartsArray) { console.log(hardCodedPartsArray); //[ 'Hello ', '!' ] console.log(replacementPartsArray); //[ 'Raja' ] let str = ''; hardCodedPartsArray.forEach((string, i) => { if (i < replacementPartsArray.length) { str += `${string} $`; } else { str += `${string} ${timeGreet()}`; //<-- append Good morning/afternoon/evening here } }); return str; } //?Usage: const firstName = 'Raja'; const greetings = greet`Hello ${firstName}!`; //??<-- Tagged literal console.log(greetings); //'Hello Raja! Good Morning!' ? function timeGreet() { const hr = new Date().getHours(); return hr < 12 ? 'Good Morning!' : hr < 18 ? 'Good Afternoon!' : 'Good Evening!'; }

Now that we discussed what “Tagged” functions are, many people want to use this feature in different domains, like in Terminal for commands and HTTP requests for composing URIs, and so on.

⚠️The problem with Tagged String literal

The problem is that ES2015 and ES2016 specs doesn’t allow using escape characters like “\u” (unicode), “\x”(hexadecimal) unless they look exactly like `\u00A9` or \u{2F804} or \xA9.

So if you have a Tagged function that internally uses some other domain’s rules (like Terminal’s rules), that may need to use \ubla123abla that doesn’t look like \u0049 or \u{@F804}, then you would get a syntax error.

In ES2018, the rules are relaxed to allow such seemingly invalid escape characters as long as the Tagged function returns the values in an object with a “cooked” property (where invalid characters are “undefined”), and then a “raw” property (with whatever you want).

function myTagFunc(str) { return { "cooked": "undefined", "raw": str.raw[0] } } var str = myTagFunc `hi \ubla123abla`; //call myTagFunc str // { cooked: "undefined", raw: "hi \\unicode" }

3. “dotall” flag for Regular expression

Currently in RegEx, although the dot(“.”) is supposed to match a single character, it doesn’t match new line characters like \n \r \f etc.

For example:

//Before /first.second/.test('first\nsecond'); //false

This enhancement makes it possible for the dot operator to match any single character. In order to ensure this doesn’t break anything, we need to use \s flag when we create the RegEx for this to work.

//ECMAScript 2018 /first.second/s.test('first\nsecond'); //true Notice: /s ?? 

Here is the overall API from the proposal doc:

4. RegExp Named Group Captures ?

This enhancement brings a useful RegExp feature from other languages like Python, Java and so on called “Named Groups.” This features allows developers writing RegExp to provide names (identifiers) in the format(?...) for different parts of the group in the RegExp. They can then use that name to grab whichever group they need with ease.

4.1 Basic Named group example

In the below example, we are using (?) (?) and (?) names to group different parts of the date RegEx. The resulting object will now contain a groups property with properties year, month , and day with corresponding values.

4.2 Using Named groups inside regex itself

We can use the \k format to back reference the group within the regex itself. The following example shows how it works.

4.3 Using named groups in String.prototype.replace

The named group feature is now baked into String’s replace instance method. So we can easily swap words in the string.

For example, change “firstName, lastName” to “lastName, firstName”.

5. Rest properties for Objects

Rest operator ... (three dots) allows us to extract Object properties that are not already extracted.

5.1 You can use rest to help extract only properties you want

5.2 Even better, you can remove unwanted items! ??

6. Spread properties for Objects

Spread properties also look just like rest properties with three dots ... but the difference is that you use spread to create (restructure) new objects.

Tip: the spread operator is used in the right side of the equals sign. The rest are used in the left-side of the equals sign.

7. RegExp Lookbehind Assertions

This is an enhancement to the RegEx that allows us to ensure some string exists immediately *before* some other string.

You can now use a group (?<=…) (question mark, less than, equals) to look behind for positive assertion.

Further, you can use (? (question mark, less than, exclamation), to look behind for a negative assertion. Essentially this will match as long as the -ve assertion passes.

Positive Assertion: Let’s say we want to ensure that the # sign exists before the word winning (that is: #winning) and want the regex to return just the string “winning”. Here is how you’d write it.

Negative Assertion: Let’s say we want to extract numbers from lines that have € signs and not $ signs before those numbers.

8. RegExp Unicode Property Escapes

It was not easy to write RegEx to match various unicode characters. Things like \w , \W , \d etc only match English characters and numbers. But what about numbers in other languages like Hindi, Greek, and so on?

That’s where Unicode Property Escapes come in. It turns out Unicode adds metadata properties for each symbol (character) and uses it to group or characterize various symbols.

For example, Unicode database groups all Hindi characters(हिन्दी) under a property called Script with value Devanagari and another property called Script_Extensions with the same value Devanagari. So we can search for Script=Devanagari and get all Hindi characters.

Devanagari can be used for various Indian languages like Marathi, Hindi, Sanskrit, and so on.

Starting in ECMAScript 2018, we can use \p to escape characters along with {Script=Devanagari} to match all those Indian characters. That is, we can use: \p{Script=Devanagari} in the RegEx to match all Devanagari characters.

//The following matches multiple hindi character /^\p{Script=Devanagari}+$/u.test('हिन्दी'); //true //PS:there are 3 hindi characters h

Similarly, Unicode database groups all Greek characters under Script_Extensions (and Script ) property with the value Greek . So we can search for all Greek characters using Script_Extensions=Greek or Script=Greek .

That is, we can use: \p{Script=Greek} in the RegEx to match all Greek characters.

//The following matches a single Greek character /\p{Script_Extensions=Greek}/u.test('π'); // true

Further, the Unicode database stores various types of Emojis under the boolean properties Emoji, Emoji_Component, Emoji_Presentation, Emoji_Modifier, and Emoji_Modifier_Base with property values as `true`. So we can search for all Emojis by simply selecting Emoji to be true.

That is, we can use: \p{Emoji} ,\Emoji_Modifier and so on to match various kinds of Emojis.

The following example will make it all clear.

//The following matches an Emoji character /\p{Emoji}/u.test('❤️'); //true //The following fails because yellow emojis don't need/have Emoji_Modifier! /\p{Emoji}\p{Emoji_Modifier}/u.test('✌️'); //false //The following matches an emoji character\p{Emoji} followed by \p{Emoji_Modifier} /\p{Emoji}\p{Emoji_Modifier}/u.test('✌?'); //true //Explaination: //By default the victory emoji is yellow color. //If we use a brown, black or other variations of the same emoji, they are considered //as variations of the original Emoji and are represented using two unicode characters. //One for the original emoji, followed by another unicode character for the color. // //So in the below example, although we only see a single brown victory emoji, //it actually uses two unicode characters, one for the emoji and another // for the brown color. // //In Unicode database, these colors have Emoji_Modifier property. //So we need to use both \p{Emoji} and \p{Emoji_Modifier} to properly and //completely match the brown emoji. /\p{Emoji}\p{Emoji_Modifier}/u.test('✌?'); //true

Lastly, we can use capital "P”(\P ) escape character instead of small p (\p ), to negate the matches.

References:

  1. ECMAScript 2018 Proposal
  2. //mathiasbynens.be/notes/es-unicode-property-escapes

8. Promise.prototype.finally()

finally() is a new instance method that was added to Promise. The main idea is to allow running a callback after either resolve or reject to help clean things up. The finally callback is called without any value and is always executed no matter what.

Let’s look at various cases.

9. Asynchronous Iteration

This is an *extremely* useful feature. Basically it allows us to create loops of async code with ease!

This feature adds a new “for-await-of” loop that allows us to call async functions that return promises (or Arrays with a bunch of promises) in a loop. The cool thing is that the loop waits for each Promise to resolve before doing to the next loop.

That’s pretty much it!

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My Other Posts

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Related ECMAScript 2015+ posts

  1. Check out these useful ECMAScript 2015 (ES6) tips and tricks
  2. 5 JavaScript “Bad” Parts That Are Fixed In ES6
  3. Is “Class” In ES6 The New “Bad” Part?