ES6 savjeti i trikovi kako biste svoj kod učinili čišćim, kraćim i lakšim za čitanje!

Predlošci literala

Literali predložaka čine rad sa stringovima mnogo lakšim nego prije. Započeli su s oznakom za povratak i mogu imati umetnute varijable pomoću ${variable}. Usporedite ove dvije linije koda:

var fName = 'Peter', sName = 'Smith', age = 43, job= 'photographer';var a = 'Hi, I\'m ' + fName + ' ' + sName + ', I\'m ' + age + ' and work as a ' + job + '.';var b = `Hi, I'm ${ fName } ${ sName }, I'm ${ age } and work as a ${ job }.`;

To čini život daleko jednostavnijim i kôd lakšim za čitanje. U kovrčave zagrade možete staviti bilo što: varijable, jednadžbe ili pozive funkcija. Koristit ću ih u primjerima kroz ovaj članak.

Opseg bloka sintakse

JavaScript je uvijek bio opseg funkcija, zbog čega je postalo uobičajeno da se cijela JavaScript datoteka umota u prazan izraz funkcije koji se odmah poziva (IIFE). To se radi za izoliranje svih varijabli u datoteci, tako da nema sukoba varijabli.

Sada imamo opseg bloka i dvije nove deklaracije varijabli koje su vezane za blok.

"Neka" deklaracija

Ovo je slično, varali ima nekoliko značajnih razlika. Budući da je blok-opsega, nova varijabla s istim imenom može se deklarirati bez utjecaja na vanjske varijable.

var a = 'car' ;{ let a = 5; console.log(a) // 5}console.log(a) // car

Budući da je vezan za opseg bloka, rješava ovo klasično pitanje u intervjuu:

"što je izlaz i kako biste ga natjerali da radi kako očekujete?"

for (var i = 1; i  { console.log(i); }, 1000);}

U ovom slučaju izlazi "5 5 5 5 5", jer se varijabla i mijenja na svakoj iteraciji.

Ako isključite varfor, lettada se sve mijenja. Sada svaka petlja stvara novi opseg bloka s vrijednošću za i koja je vezana za tu petlju. Iako ste napisali:

{let i = 1; setTimeout(() => { console.log(i) }, 1000)} {let i = 2; setTimeout(() => { console.log(i) }, 1000)} {let i = 3; setTimeout(() => { console.log(i) }, 1000)} {let i = 4; setTimeout(() => { console.log(i) }, 1000)} {let i = 5; setTimeout(() => { console.log(i) }, 1000)} 

Druga razlika između vari letje ta što letse ne podiže onako kako varjest.

{ console.log(a); // undefined console.log(b); // ReferenceError var a = 'car'; let b = 5;}

Zbog strožeg opsega i predvidljivijeg ponašanja, neki su ljudi rekli da biste trebali koristiti letumjesto var, osim tamo gdje vam je posebno potrebno podizanje ili labaviji opseg vardeklaracije.

Konst

Ako ste prije htjeli proglasiti konstantnu varijablu u JavaScript-u, bilo je uobičajeno da se varijabla imenuje u velika slova. Međutim, ovo ne bi osiguralo varijablu - samo je dalo na znanje drugim programerima da je to konstanta i da je ne treba mijenjati.

Sad imamo constdeklaraciju.

{ const c = "tree"; console.log(c); // tree c = 46; // TypeError! }

constne čini varijablu nepromjenjivom, samo zaključava njezinu dodjelu. Ako imate složeni zadatak (objekt ili niz), tada se vrijednost može i dalje mijenjati.

{ const d = [1, 2, 3, 4]; const dave = { name: 'David Jones', age: 32}; d.push(5); dave.job = "salesman"; console.log(d); // [1, 2, 3, 4, 5] console.log(dave); // { age: 32, job: "salesman", name: 'David Jones'}}

Problem s funkcijama opsega blokova

Deklaracije funkcija sada su određene kako bi bile vezane za blokiranje opsega.

{ bar(); // works function bar() { /* do something */ }}bar(); // doesn't work

Problem dolazi kada deklarirate funkciju unutar ifizraza.

Uzmite u obzir ovo:

if ( something) { function baz() { console.log('I passed') }} else { function baz() { console.log('I didn\'t pass') } } baz();

Prije ES6, obje deklaracije funkcije bile bi podignute i rezultat bi bio 'I didn\'t pass'bez obzira što somethingbio.

Sada dobivamo 'ReferenceError', kao što bazje uvijek vezano opsegom bloka.

Širenje

ES6 uvodi ...operatora, koji se naziva 'operator širenja'. Ima dvije glavne namjene: širenje niza ili objekta u novi niz ili objekt i spajanje više parametara u niz.

Prvi je slučaj upotrebe onaj s kojim ćete se vjerojatno najviše susresti, pa ćemo to prvo razmotriti.

let a = [3, 4, 5];let b = [1, 2, ...a, 6];console.log(b); // [1, 2, 3, 4, 5, 6]

To može biti vrlo korisno za prosljeđivanje skupa varijabli funkciji iz niza.

function foo(a, b, c) { console.log(`a=${a}, b=${b}, c=${c}`)} let data = [5, 15, 2];foo( ...data); // a=5, b=15, c=2

Objekt se također može širiti unosom svakog od parova vrijednosti ključa u novi objekt. (Širenje predmeta zapravo je u fazi 4 prijedloga i službeno će biti u ES2018. Podržava ga samo Chrome 60 ili noviji, Firefox 55 ili noviji i Node 6.4.0 ili noviji)

let car = { type: 'vehicle ', wheels: 4};let fordGt = { make: 'Ford', ...car, model: 'GT'};console.log(fordGt); // {make: 'Ford', model: 'GT', type: 'vehicle', wheels: 4}

Druga značajka operatora širenja je da stvara novi niz ili objekt. Primjer u nastavku stvara novi niz za b, ali se csamo odnosi na isti niz.

let a = [1, 2, 3];let b = [ ...a ];let c = a;b.push(4);console.log(a); // [1, 2, 3]console.log(b); // [1, 2, 3, 4] referencing different arraysc.push(5);console.log(a); // [1, 2, 3, 5] console.log(c); // [1, 2, 3, 5] referencing the same array

Drugi je slučaj skupljanja varijabli u niz. Ovo je vrlo korisno kada ne znate koliko se varijabli prenosi funkciji.

function foo(...args) { console.log(args); } foo( 'car', 54, 'tree'); // [ 'car', 54, 'tree' ] 

Zadani parametri

Funkcije se sada mogu definirati sa zadanim parametrima. Nedostale ili nedefinirane vrijednosti inicijaliziraju se sa zadanom vrijednošću. Samo budite oprezni - jer su null i false vrijednosti prisiljene na 0.

function foo( a = 5, b = 10) { console.log( a + b);} foo(); // 15foo( 7, 12 ); // 19foo( undefined, 8 ); // 13foo( 8 ); // 18foo( null ); // 10 as null is coerced to 0

The default values can be more than just values — they can also be expressions or functions.

function foo( a ) { return a * 4; }function bar( x = 2, y = x + 4, z = foo(x)) { console.log([ x, y, z ]);}bar(); // [ 2, 6, 8 ]bar( 1, 2, 3 ); //[ 1, 2, 3 ] bar( 10, undefined, 3 ); // [ 10, 14, 3 ]

Destructuring

Destructuring is the process of taking apart the array or object on the left hand side of the equal sign. The array or object can come from a variable, function, or equation.

let [ a, b, c ] = [ 6, 2, 9];console.log(`a=${a}, b=${b}, c=${c}`); //a=6, b=2, c=9

function foo() { return ['car', 'dog', 6 ]; } let [ x, y, z ] = foo();console.log(`x=${x}, y=${y}, z=${z}`); // x=car, y=dog, z=6

With object destructuring, the keys of the object can be listed inside curly braces to extract that key-value pair.

function bar() { return {a: 1, b: 2, c: 3}; }let { a, c } = bar();console.log(a); // 1console.log(c); // 3console.log(b); // undefined

Sometimes, you want to extract the values but assign them to a new variable. This is done using a 'key: variable' pairing on the left of the equals sign.

function baz() { return { x: 'car', y: 'London', z: { name: 'John', age: 21} }; }let { x: vehicle, y: city, z: { name: driver } } = baz();

console.log( `I'm going to ${city} with ${driver} in their ${vehicle}.`); // I'm going to London with John in their car. 

Another thing that object destructuring allows is assigning a value to multiple variables.

let { x: first, x: second } = { x: 4 };console.log( first, second ); // 4, 4

Object Literals and Concise Parameters

When you are creating an object literal from variables, ES6 allows you to omit the key if it is the same as the variable name.

let a = 4, b = 7;let c = { a: a, b: b };let concise = { a, b };console.log(c, concise) // {a: 4, b: 7}, {a: 4, b: 7}

This can also be used in combination with destructuring to make your code much simpler and cleaner.

function foo() { return { name: 'Anna', age: 56, job: { company: 'Tesco', title: 'Manager' } };} 

// pre ES6let a = foo(), name = a.name, age = a.age, company = a.job.company;

// ES6 destructuring and concise parameters let { name, age, job: {company}} = foo();

It can also be used to destructure objects passed into functions. Method 1 and 2 are how you would have done it before ES6, and method 3 uses destructuring and concise parameters.

let person = { name: 'Anna', age: 56, job: { company: 'Tesco', title: 'Manager' }};

// method 1function old1( person) { var yearOfBirth = 2018 - person.age; console.log( `${ person.name } works at ${ person.job.company } and was born in ${ yearOfBirth }.`);}

// method 2function old1( person) { var age = person.age, yearOfBirth = 2018 - age, name = person.name, company = person.job.company; console.log( `${ name } works at ${ company } and was born in ${ yearOfBirth }.`);} 

// method 3function es6({ age, name, job: {company}}) { var yearOfBirth = 2018 - age; console.log( `${ name } works at ${ company } and was born in ${ yearOfBirth }.`);} 

Using ES6, we can extract the age, name and company without extra variable declaration.

Dynamic Property Names

ES6 adds the ability to create or add properties with dynamically assigned keys.

let city= 'sheffield_';let a = { [ city + 'population' ]: 350000};a[ city + 'county' ] = 'South Yorkshire';console.log(a); // {sheffield_population: 350000, sheffield_county: 'South Yorkshire' }

Arrow Functions

Arrow functions have two main aspects: their structure and their this binding.

They can have a much simpler structure than traditional functions because they don't need the function key word, and they automatically return whatever is after the arrow.

var foo = function( a, b ) { return a * b;} 

let bar = ( a, b ) => a * b;

If the function requires more than a simple calculation, curly braces can be used and the function returns whatever is returned from the block scope.

let baz = ( c, d ) => { let length = c.length + d.toString().length; let e = c.join(', '); return `${e} and there is a total length of ${length}`;}

One of the most useful places for arrow functions is in array functions like .map, .forEach or .sort.

let arr = [ 5, 6, 7, 8, 'a' ];let b = arr.map( item => item + 3 );console.log(b); // [ 8, 9, 10, 11, 'a3' ]

As well as having a shorter syntax, it also fixes the issues that often arose around the this binding behaviour. The fix with pre-ES6 functions was to store the this reference, often as a self variable.

var clickController = { doSomething: function (..) { var self = this; btn.addEventListener( 'click', function() { self.doSomething(..) }, false ); } };

This had to be done because the this binding is dynamic. This means that the this inside the event listener and the this inside the doSomething do not refer to the same thing.

Inside arrow functions, the this binding is lexical, not dynamic. This was the main design feature of the arrow function.

Whilst lexical this binding can be great, sometimes that's not what is wanted.

let a = { oneThing: ( a ) => { let b = a * 2; this.otherThing(b); }, otherThing: ( b ) => {....} };

a.oneThing(6);

When we use a.oneThing(6), the this.otherThing( b ) reference fails as this doesn't point to the a object, but to the surrounding scope. If you are rewriting legacy code using ES6 syntax, this is something to watch out for.

for … of Loops

ES6 adds a way to iterate over each of the values in an array. This is different from the existing for ... in loop that loops over the key/index.

let a = ['a', 'b', 'c', 'd' ];// ES6 for ( var val of a ) { console.log( val );} // "a" "b" "c" "d"// pre-ES6 for ( var idx in a ) { console.log( idx );} // 0 1 2 3

Using the new for … of loop saves adding a let val = a[idx] inside each loop.

Arrays, strings, generators and collections are all iterable in standard JavaScript. Plain objects can't normally be iterated over, unless you have defined an iterator for it.

Number Literals

ES5 code handled decimal and hexadecimal number formats well, but octal form wasn't specified. In fact, it was actively disallowed in strict mode.

ES6 has added a new format, adding an o after the initial 0 to declare the number an octal. They've also added a binary format.

Number( 29 ) // 29Number( 035 ) // 35 in old octal form. Number( 0o35 ) // 29 in new octal form Number( 0x1d ) // 29 in hexadecimal Number( 0b11101 ) // 29 in binary form

And Much More…

There is much, much more that ES6 offers us to make our code cleaner, shorter, easier to read and more robust. I aim to write a continuation of this article covering the less well known bits of ES6.

If you can’t wait that long, have a read of Kyle Simpson’s You Don’t Know JS book on ES6, or check out this brilliant little website!

Do you want to become a developer and get your first software job? Download the 7 Steps to Becoming a Developer and Getting Your First Job.

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