旧的 JS 版本以数字命名:ES5 (2009) 和 ES6 (2015)。
从 2016 年开始,版本按年份命名:ECMAScript 2016、2017、2018、2019,...
本章介绍 ECMAScript 2017 中的新功能:
JavaScript 字符串填充
JavaScript 对象条目()
JavaScript 对象值()
JavaScript 异步和等待
JavaScript Object.getOwnPropertyDescriptors
ECMAScript 2017 向 JavaScript 添加了两个字符串方法: padStart()
和 padEnd()
支持字符串开头和结尾的填充。
let text = "5";
text = text.padStart(4,0);
自己尝试一下 →
<!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);
自己尝试一下 →
<!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>
自 2017 年 4 月起,所有现代浏览器都支持 JavaScript 字符串填充:
Chrome 57 | Edge 15 | Firefox 48 | Safari 10 | Opera 44 |
Mar 2017 | Apr 2017 | Aug 2016 | Sep 2016 | Mar 2017 |
ECMAScript 2017 向对象添加了 Object.entries()
方法。
Object.entries()
返回对象中键/值对的数组:
const person = {
firstName : "John",
lastName : "Doe",
age : 50,
eyeColor : "blue"
};
let text = Object.entries(person);
自己尝试一下 →
<!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()
使在循环中使用对象变得简单:
const fruits = {Bananas:300, Oranges:200, Apples:500};
let text = "";
for (let [fruit, value] of Object.entries(fruits)) {
text += fruit + ": " + value + "<br>";
}
自己尝试一下 →
<!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()
还使得将对象转换为地图变得简单:
const fruits = {Bananas:300, Oranges:200, Apples:500};
const myMap = new Map(Object.entries(fruits));
自己尝试一下 →
<!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>
自 2017 年 3 月起,所有现代浏览器均支持 Object.entries()
:
Chrome 47 | Edge 14 | Firefox 47 | Safari 10.1 | Opera 41 |
Jun 2016 | Aug 2016 | Jun 2016 | Mar 2017 | Oct 2016 |
Object.values()
与 Object.entries()
类似, 但返回对象值的一维数组:
const person = {
firstName : "John",
lastName : "Doe",
age : 50,
eyeColor : "blue"
};
let text = Object.values(person);
自己尝试一下 →
<!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>
自 2017 年 3 月起,所有现代浏览器均支持 Object.values()
:
Chrome 54 | Edge 14 | Firefox 47 | Safari 10.1 | Opera 41 |
Oct 2016 | Aug 2016 | Jun 2016 | Mar 2017 | Oct 2016 |
async function myDisplay() {
let myPromise = new Promise(function(myResolve, myReject) {
setTimeout(function() { myResolve("I love You !!"); }, 3000);
});
document.getElementById("demo").innerHTML = await myPromise;
}
myDisplay();
自己尝试一下 →
<!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 和 Chrome 是第一批支持异步 JavaScript 函数的浏览器:
Chrome 55 | Edge 15 | Firefox 52 | Safari 11 | Opera 42 |
Dec 2016 | Apr 2017 | Mar 2017 | Sep 2017 | Dec 2016 |