JavaScript ECMAScript 2017

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JavaScript-versionsnumre

Gamle JS-versioner er navngivet med numre: ES5 (2009) og ES6 (2015).

Fra 2016 er versionerne navngivet efter år: ECMAScript 2016, 2017, 2018, 2019, ...

Nye funktioner i ECMAScript 2017

Dette kapitel introducerer de nye funktioner i ECMAScript 2017:

JavaScript streng polstring
JavaScript-objektposter()
JavaScript-objektværdier()
JavaScript asynkron og afvent
JavaScript Object.getOwnPropertyDescriptors

JavaScript-strengpolstring

ECMAScript 2017 føjede to strengmetoder til JavaScript: padStart() og padEnd() for at understøtte udfyldning i begyndelsen og slutningen af en streng.

Eksempler

let text = "5";
text = text.padStart(4,0);

Prøv det selv →

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Strings</h1>
<h2>The padStart() Method</h2>

<p>The padStart() method pads a string from the start.</p>
<p>It pads the string with another string (multiple times) until it reaches a given length.</p>

<p id="demo"></p>

<script>
let text = "5";
text = text.padStart(4,"0");

document.getElementById("demo").innerHTML = text;
</script>

</body>
</html>

let text = "5";
text = text.padEnd(4,0);

Prøv det selv →

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Strings</h1>
<h2>The padEnd() Method</h2>

<p>The padEnd() method pads a string at the end.</p>
<p>It pads the string with another string (multiple times) until it reaches a given length.</p>

<p id="demo"></p>

<script>
let text = "5";
text = text.padEnd(4,"0");

document.getElementById("demo").innerHTML = text;
</script>

</body>
</html>

JavaScript-strengpolstring er understøttet i alle moderne browsere siden april 2017:


Chrome 57	Edge 15	Firefox 48	Safari 10	Opera 44
Mar 2017	Apr 2017	Aug 2016	Sep 2016	Mar 2017

JavaScript-objektindgange

ECMAScript 2017 føjede metoden Object.entries() til objekter.

Object.entries() returnerer en matrix af nøgle/værdi-parrene i et objekt:

Eksempel

const person = {
  firstName : "John",
  lastName : "Doe",
  age : 50,
  eyeColor : "blue"
};

let text = Object.entries(person);

Prøv det selv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>The Object.entries() method returns an array of the key/value pairs in an object:</p>

<p id="demo"></p>

<script>
const person = {
  firstName : "John",
  lastName  : "Doe",
  age     : 50,
  eyeColor  : "blue"
};

let text = Object.entries(person);
document.getElementById("demo").innerHTML = text;
</script>

</body>
</html>

Object.entries() gør det nemt at bruge objekter i sløjfer:

Eksempel

const fruits = {Bananas:300, Oranges:200, Apples:500};

let text = "";
for (let [fruit, value] of Object.entries(fruits)) {
  text += fruit + ": " + value + "<br>";
}

Prøv det selv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>Object.entries() makes it simple to use objects in loops:</p>

<p id="demo"></p>

<script>
const fruits = {Bananas:300, Oranges:200, Apples:500}; 

let text = "";
for (let [fruit, amount] of Object.entries(fruits)) {
  text += fruit + ": " + amount + "<br>";
}

document.getElementById("demo").innerHTML = text;
</script>

</body>
</html>

Object.entries() gør det også nemt at konvertere objekter til kort:

Eksempel

const fruits = {Bananas:300, Oranges:200, Apples:500};

const myMap = new Map(Object.entries(fruits));

Prøv det selv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>Object.entries() makes it simple to convert Object to Map:</p>

<p id="demo"></p>

<script>
const fruits = {Bananas:300, Oranges:200, Apples:500}; 

const myMap = new Map(Object.entries(fruits));

document.getElementById("demo").innerHTML = myMap;
</script>

</body>
</html>

Object.entries() er understøttet i alle moderne browsere siden marts 2017:


Chrome 47	Edge 14	Firefox 47	Safari 10.1	Opera 41
Jun 2016	Aug 2016	Jun 2016	Mar 2017	Oct 2016

JavaScript-objektværdier

Object.values() ligner Object.entries(), men returnerer en enkelt dimensionsmatrix af objektværdierne:

Eksempel

const person = {
  firstName : "John",
  lastName : "Doe",
  age : 50,
  eyeColor : "blue"
};

let text = Object.values(person);

Prøv det selv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object Methods</h2>

<p>The Object.values() method returns an array of values from an object:</p>

<p id="demo"></p>

<script>
const person = {
  firstName : "John",
  lastName  : "Doe",
  age     : 50,
  eyeColor  : "blue"
};

let text = Object.values(person)
document.getElementById("demo").innerHTML = text;
</script>

</body>
</html>

Object.values() er understøttet i alle moderne browsere siden marts 2017:


Chrome 54	Edge 14	Firefox 47	Safari 10.1	Opera 41
Oct 2016	Aug 2016	Jun 2016	Mar 2017	Oct 2016

JavaScript Async funktioner

Venter på en timeout

async function myDisplay() {
  let myPromise = new Promise(function(myResolve,  myReject) {
    setTimeout(function() { myResolve("I love You !!"); }, 3000);
  });
  document.getElementById("demo").innerHTML = await myPromise;
}

 myDisplay();

Prøv det selv →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript async / await</h2>

<p>Wait 3 seconds (3000 milliseconds) for this page to change.</p>

<h1 id="demo"></h1>

<script>
async function myDisplay() {
  let myPromise = new Promise(function(resolve) {
    setTimeout(function() {resolve("I love You !!");}, 3000);
  });
  document.getElementById("demo").innerHTML = await myPromise;
}

myDisplay();
</script>

</body>
</html>

Firefox og Chrome var de første browsere med understøttelse af asynkrone JavaScript-funktioner:


Chrome 55	Edge 15	Firefox 52	Safari 11	Opera 42
Dec 2016	Apr 2017	Mar 2017	Sep 2017	Dec 2016