Her er eksempler på alt nyt i ECMAScript 2016, 2017 og 2018

Det er svært at holde styr på, hvad der er nyt i JavaScript (ECMAScript). Og det er endnu sværere at finde nyttige kodeeksempler.

Så i denne artikel vil jeg dække alle 18 funktioner, der er anført i TC39s færdige forslag, der blev tilføjet i ES2016, ES2017 og ES2018 (endelig udkast) og viser dem med nyttige eksempler.

Dette er et ret langt indlæg, men skal være let at læse. Tænk på dette som " Netflix binge-læsning." I slutningen af ​​dette lover jeg, at du har masser af viden om alle disse funktioner.

OK, lad os gå over disse en efter en.

1. Array.prototype.includes

includeser en simpel instansmetode på Array og hjælper med let at finde ud af, om et element er i Array (inklusive NaNulig indexOf).

Trivia: JavaScript-spec-folk ønskede at navngive det contains, men dette blev tilsyneladende allerede brugt af Mootools, så de brugte includes.

2. Eksponentiering infix operator

Matematiske operationer som addition og subtraktion har infix-typen operatorer såsom +og -hhv. På samme måde som dem **bruges infix-operatøren ofte til eksponentdrift. I ECMAScript 2016 blev den ** introduceret i stedet for Math.pow.

1. Objektværdier ()

Object.values()er en ny funktion, der svarer til, Object.keys()men returnerer alle værdierne for Objektets egne egenskaber eksklusive enhver værdi (r) i den prototypiske kæde.

2. Object.entries ()

Object.entries()er relateret til Object.keys, men i stedet for at returnere bare nøgler, returnerer det både nøgler og værdier i array-mode. Dette gør det meget simpelt at gøre ting som at bruge objekter i sløjfer eller konvertere objekter til Maps.

Eksempel 1:

Eksempel 2:

3. Strengpolstring

To instansmetoder blev føjet til String - String.prototype.padStart og String.prototype.padEnd- der tillader tilføjelse / forudgående enten en tom streng eller en anden streng til starten eller slutningen af ​​den originale streng.

'someString'.padStart(numberOfCharcters [,stringForPadding]); '5'.padStart(10) // ' 5' '5'.padStart(10, '=*') //'=*=*=*=*=5' '5'.padEnd(10) // '5 ' '5'.padEnd(10, '=*') //'5=*=*=*=*='
Dette er praktisk, når vi vil justere ting i scenarier som smuk udskrivningsvisning eller terminaludskrivning.

3.1 padStarteksempel:

I nedenstående eksempel har vi en liste over antal af forskellige længder. Vi ønsker at sætte "0" på forhånd, så alle elementerne har den samme længde på 10 cifre til visningsformål. Vi kan bruge padStart(10, '0')til let at opnå dette.

3.2 padEnd eksempel:

padEnd virkelig kommer godt med, når vi udskriver flere emner med forskellige længder og ønsker at justere dem korrekt.

Nedenstående eksempel er en god realistisk eksempel på, hvordan padEnd, padStartog Object.entrieskommer alle sammen til at producere en smuk output.

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 and padEnd på Emojis og andre dobbeltbyte tegn

Emojis og andre dobbeltbyte-tegn er repræsenteret ved hjælp af flere byte af unicode. Så padStart og padEnd fungerer muligvis ikke som forventet! ⚠️

For eksempel: Lad os sige, at vi prøver at blokere strengen for heartat nå 10tegn med ❤️ emoji. Resultatet ser ud som nedenfor:

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

Dette skyldes, at ❤️ er 2 kodepunkter lange ( '\u2764\uFE0F')! Selve ordet hearter 5 tegn, så vi har kun i alt 5 tegn tilbage. Så hvad der sker er, at JS puder to hjerter ved hjælp af, '\u2764\uFE0F'og det producerer ❤️❤️. For den sidste bruger den simpelthen den første byte i hjertet, \u2764der producerer ❤

Så vi ender med: ❤️❤️❤heart

PS: Du kan bruge dette link til at tjekke unicode char-konverteringer.

4. Object.getOwnPropertyDescriptors

Denne metode returnerer alle detaljer (inklusive getter- getog settermetoder set) for alle egenskaberne for et givet objekt. Hovedmotivationen til at tilføje dette er at tillade lav kopiering / kloning af et objekt til et andet objektder også kopierer getter- og setterfunktioner i modsætning til Object.assign.

Object.assign overfladisk kopierer alle detaljer undtagen getter- og setterfunktionerne for det originale kildeobjekt.

Eksemplet nedenfor viser forskellen mellem Object.assignog Object.getOwnPropertyDescriptorssammen med Object.definePropertiesat kopiere et originalt objekt Cartil et nyt objekt ElectricCar. Du vil se, at ved at bruge Object.getOwnPropertyDescriptors, discounter getter og setter funktioner også kopieret ind i målet objekt.

FØR…

EFTER…

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

Dette er en mindre opdatering, der giver os mulighed for at have efterfølgende kommaer efter den sidste funktionsparameter. Hvorfor? At hjælpe med værktøjer som git-skyld for at sikre, at kun nye udviklere får skylden.

Nedenstående eksempel viser problemet og løsningen.

Bemærk: Du kan også ringe til funktioner med efterfølgende kommaer!

6. Async / afventer

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

Da hvert awaitudtryk returnerer et løfte, kan du fange fejl på hver linje som vist nedenfor.

//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); }); }

Mulighed 3 - Fang hele async-await-funktionen

//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 er i øjeblikket i det endelige udkast og vil være ude i juni eller juli 2018. Alle nedenstående funktioner er i fase 4 og vil være en del af ECMAScript 2018.

1. Delt hukommelse og atomik

Dette er en enorm, ret avanceret funktion og er en kerneforbedring af JS-motorer.

Hovedideen er at bringe en slags multi-threading-funktion til JavaScript, så JS-udviklere kan skrive højtydende, samtidige programmer i fremtiden ved at tillade at administrere hukommelse alene i stedet for at lade JS-motoren styre hukommelsen.

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.

Men at dele hukommelse mellem tråde kan forårsage raceforhold. For at undgå raceforhold introduceres det globale objekt “ Atomics ”. Atomics giver forskellige metoder til at låse den delte hukommelse, når en tråd bruger dens data. Det giver også metoder til at opdatere sådanne data i den delte hukommelse sikkert.

Anbefalingen er at bruge denne funktion via et bibliotek, men lige nu er der ingen biblioteker bygget oven på denne funktion.

Hvis du er interesseret, anbefaler jeg at læse:

  1. Fra arbejdere til delt erindring y - lucasfcosta
  2. En tegneserie intro til SharedArrayBuffers - Lin Clark
  3. Delt hukommelse og atomik - Dr. Axel Rauschmayer

2. Tagget skabelon bogstavelig begrænsning fjernet

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!

If this was useful, please click the clap ? button down below a few times to show your support! ⬇⬇⬇ ??

My Other Posts

//medium.com/@rajaraodv/latest

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?