ecmascript 6 features - part 2

,

Continued from Part 1

A brief overview of some of the latest features available in ECMAScript 6, with examples of how they were written previously in ECMAScript 5, if even possible.

Symbol Type

Symbol Type

Unique and immutable data type to be used as an identifier for object properties. Symbol can have an optional description, but only for debugging purposes.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
Symbol("foo") !== Symbol("foo")
const foo = Symbol()
const bar = Symbol()
typeof foo === "symbol"
typeof bar === "symbol"
let obj = {}
obj[foo] = "foo"
obj[bar] = "bar"
JSON.stringify(obj) // {}
Object.keys(obj) // []
Object.getOwnPropertyNames(obj) // []
Object.getOwnPropertySymbols(obj) // [ foo, bar ]

ECMAScript 5

1
// no equivalent in ES5

Global Symbols

Global symbols indexed through unique keys.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
Symbol.for("app.foo") === Symbol.for("app.foo")
const foo = Symbol.for("app.foo")
const bar = Symbol.for("app.bar")
Symbol.keyFor(foo) === "app.foo"
Symbol.keyFor(bar) === "app.bar"
typeof foo === "symbol"
typeof bar === "symbol"
let obj = {}
obj[foo] = "foo"
obj[bar] = "bar"
JSON.stringify(obj) // {}
Object.keys(obj) // []
Object.getOwnPropertyNames(obj) // []
Object.getOwnPropertySymbols(obj) // [ foo, bar ]

ECMAScript 5

1
// no equivalent in ES5

Iterators

Iterator & For-Of Operator

Support “interable” protocol to allow objects to customize their iteration behaviour. Additionally, support “iterator” protocol to produce sequence of values, either finite or infinite. Finally, provide convenient of-operator to iterate over all values of an iterable object.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
let fibonacci = {
    [Symbol.iterator]() {
        let pre = 0, cur = 1
        return {
            next () {
                [ pre, cur ] = [ cur, pre + cur ]
                return { done: false, value: cur }
            }
        }
    }
}

for (let n of fibonacci) {
    if (n > 1000)
        break
    console.log(n)
}

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
var fibonacci = {
    next: (function () {
        var pre = 0, cur = 1;
        return function () {
            tmp = pre;
            pre = cur;
            cur += tmp;
            return cur;
        };
    })()
};

var n;
for (;;) {
    n = fibonacci.next();
    if (n > 1000)
        break;
    console.log(n);
}

Generators

Generator Function, Iterator Protocol

Support for generators, a special case of iterators containing a generator function, where the control flow can be paused and resumed, in order to produce a sequence of values (either finite or infinite).

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
let fibonacci = {
    *[Symbol.iterator]() {
        let pre = 0, cur = 1
        for (;;) {
            [ pre, cur ] = [ cur, pre + cur ]
            yield cur
        }
    }
}

for (let n of fibonacci) {
    if (n > 1000)
        break
    console.log(n)
}

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
var fibonacci = {
    next: (function () {
        var pre = 0, cur = 1;
        return function () {
            tmp = pre;
            pre = cur;
            cur += tmp;
            return cur;
        };
    })()
};

var n;
for (;;) {
    n = fibonacci.next();
    if (n > 1000)
        break;
    console.log(n);
}

Generator Function, Direct Use

Support for generator functions, a special variant of functions, where the control flow can be paused and resumed in order to produce a sequence of values (either finite or infinite).

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
function* range (start, end, step) {
    while (start < end) {
        yield start
        start += step
    }
}

for (let i of range(0, 10, 2)) {
    console.log(i) // 0, 2, 4, 6, 8
}

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
11
12
13
function range (start, end, step) {
    var list = [];
    while (start < end) {
        list.push(start);
        start += step;
    }
    return list;
}

var r = range(0, 10, 2);
for (var i = 0; i < r.length; i++) {
    console.log(r[i]); // 0, 2, 4, 6, 8
}

Generator Matching

Generator functions can produce and spread a sequence of values (either finite or infinite).

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
let fibonacci = function* (numbers) {
    let pre = 0, cur = 1
    while (numbers-- > 0) {
        [ pre, cur ] = [ cur, pre + cur ]
        yield cur
    }
}

for (let n of fibonacci(1000))
    console.log(n)

let numbers = [ ...fibonacci(1000) ]
let [ n1, n2, n3, ...others ] = fibonacci(1000)

ECMAScript 5

1
// no equivalent in ES5

Generator Control-Flow

Generator functions can support asynchronous programming in the style of “co-routines” in combination with promises.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
// generic asynchonous control-flow driver
function async (proc, ...params) {
    let iterator = proc(...params)
    return new Promise((resolve, reject) => {
        let loop = (value) => {
            let result
            try {
                result = iterator.next(value)
            }
            catch (err) {
                reject(err)
            }
            if (result.done)
                resolve(result.value)
            else if (typeof result.value === "object" && typeof result.value.then === "function")
                result.value.then((value) => {
                    loop(value)
                }, (err) => {
                    reject(err)
                })
            else
                loop(result.value)
        }
        loop()
    })
}

// application-specific asynchronous builder
function makeAsync (text, after) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(text), after)
    })
}

// application-specific asynchronous procedure
async(function* (greeting) {
    let foo = yield makeAsync("foo", 300)
    let bar = yield makeAsync("bar", 200)
    let baz = yield makeAsync("baz", 100)
    return `${greeting} ${foo} ${bar} ${baz}`
}, "Hello").then((msg) => {
    console.log("RESULT:", msg) // "Hello foo bar baz"
})

ECMAScript 5

1
// no equivalent in ES5

Generator Methods

Support for methods in classes and on objects, based on generator functions

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
class Clz {
    * bar () {
        ...
    }
}
let Obj = {
    * foo () {
        ...
    }
}

ECMAScript 5

1
// no equivalent in ES5

Map/Set & WeakMap/WeakSet

Set Data-Structure

Cleaner data-structure for common algorithms based on sets.

ECMAScript 6

1
2
3
4
5
6
let s = new Set()
s.add("hello").add("goodbye").add("hello")
s.size === 3
s.has("hello") === true
for (let key of s.values()) // insertion order
    console.log(key)

ECMAScript 5

1
2
3
4
5
6
7
var s = {};
s["hello"] = true; s["goodbye"] = true; s["hello"] = true;
Object.keys(s).length === 2
s["hello"] === true;
for (var key in s) // arbitrary order
    if (s.hasOwnProperty(key))
        console.log(s[key]);

Map Data-Structure

Cleaner data-structure for common algorithms based on maps.

ECMAScript 6

1
2
3
4
5
6
7
8
let m = new Map()
let s = Symbol()
m.set("hello", 42)
m.set(s, 34)
m.get(s) === 34
m.size === 2
for (let [ key, val ] of m.entries())
    console.log(key + " = " + val)

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
11
12
var m = {};
// no equivalent in ES5
m["hello"] = 42;
// no equivalent in ES5
// no equivalent in ES5
Objects.keys(m).length === 2;
for (key in m) {
    if (m.hasOwnProperty(key)) {
        var val = m[key];
        console.log(key + " = " + val);
    }
}

Memory-leak-free Object-key’d side-by-side data-structures.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
let isMarked = new WeakSet()
let attachedData = new WeakMap()

export class Node {
    constructor () { this.id = id }
    mark () { isMarked.add(this) }
    unmark () { isMarked.delete(this) }
    marked () { return isMarked.has(this) }
    set data (data) { attachedData.set(this, data) }
    get data () { return attachedData.get(this) }
}

let foo = new Node("foo")

JSON.stringify(foo) === '{ "id": "foo" }'
foo.mark()
foo.data = "bar"
foo.data === "bar"
JSON.stringify(foo) === '{ "id": "foo" }'

isMarked.has(foo) === true
attachedData.has(foo) === true
foo = null /* remove only reference to foo */
attachedData.has(foo) === false
isMarked.has(foo) === false

ECMAScript 5

1
// no equivalent in ES5

Typed Arrays

Typed Arrays

Support for arbitrary byte-based data structures to implement network protocols, cryptography algorithms, file format manipulations, etc.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
// ES6 class equivalent to the following C structure:
// struct Example { unsigned long id; char username[16]; float amountDue }
class Example {
    constructor (buffer = new ArrayBuffer(24)) {
        this.buffer = buffer
    }
    set buffer (buffer) {
        this._buffer = buffer
        this._id = new Uint32Array (this._buffer, 0, 1)
        this._username = new Uint8Array (this._buffer, 4, 16)
        this._amountDue = new Float32Array(this._buffer, 20, 1)
    }
    get buffer () { return this._buffer }
    set id (v) { this._id[0] = v }
    get id () { return this._id[0] }
    set username (v) { this._username[0] = v }
    get username () { return this._username[0] }
    set amountDue (v) { this._amountDue[0] = v }
    get amountDue () { return this._amountDue[0] }
}

let example = new Example()
example.id = 7
example.username = "John Doe"
example.amountDue = 42.0

ECMAScript 5

1
2
// no equivalent in ES5
// (only an equivalent in HTML5)

New Build-In Methods

Object Property Assignment

New function for assigning enumerable properties of one or more source objects onto a destination object.

ECMAScript 6

1
2
3
4
5
6
7
8
var dest = { quux: 0 }
var src1 = { foo: 1, bar: 2 }
var src2 = { foo: 3, baz: 4 }
Object.assign(dest, src1, src2)
dest.quuz === 0
dest.foo === 3
dest.bar === 2
dest.baz === 4

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
11
12
13
var dest = { quux: 0 };
var src1 = { foo: 1, bar: 2 };
var src2 = { foo: 3, baz: 4 };
Object.keys(src1).forEach(function(k) {
    dest[k] = src1[k];
});
Object.keys(src2).forEach(function(k) {
    dest[k] = src2[k];
});
dest.quux === 0;
dest.foo  === 3;
dest.bar  === 2;
dest.baz  === 4;

Array Element Finding

New function for finding an element in an array.

ECMAScript 6

1
2
[ 1, 3, 4, 2 ].find(x => x > 3) // 4
[ 1, 3, 4, 2 ].findIndex(x => x > 3) // 2

ECMAScript 5

1
2
[ 1, 3, 4, 2 ].filter(function (x) { return x > 3 })[0]; // 4
// no equivalent in ES5

String Repeating

New string repeating functionality.

ECMAScript 6

1
2
" ".repeat(4 * depth)
"foo".repeat(3)

ECMAScript 5

1
2
Array((4 * depth) + 1).join(" ");
Array(3 + 1).join("foo");

String Searching

New specific string functions to search for a sub-string.

ECMAScript 6

1
2
3
4
5
"hello".startsWith("ello", 1) // true
"hello".endsWith("hell", 4) // true
"hello".includes("ell") // true
"hello".includes("ell", 1) // true
"hello".includes("ell", 2) // false

ECMAScript 5

1
2
3
4
5
"hello".indexOf("ello") === 1; // true
"hello".indexOf("hell") === (4 - "hell".length); // true
"hello".indexOf("ell") !== -1; // true
"hello".indexOf("ell", 1) !== -1; // true
"hello".indexOf("ell", 2) !== -1; // false

Number Type Checking

New functions for checking for non-numbers and finite numbers.

ECMAScript 6

1
2
3
4
5
6
7
Number.isNaN(42) === false
Number.isNaN(NaN) === true

Number.isFinite(Infinity) === false
Number.isFinite(-Infinity) === false
Number.isFinite(NaN) === false
Number.isFinite(123) === true

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
11
12
13
var isNaN = function (n) {
    return n !== n;
};
var isFinite = function (v) {
    return (typeof v === "number" && !isNaN(v) && v !== Infinity && v !== -Infinity);
};
isNaN(42) === false;
isNaN(NaN) === true;

isFinite(Infinity) === false;
isFinite(-Infinity) === false;
isFinite(NaN) === false;
isFinite(123) === true;

Number Safety Checking

Checking whether an integer number is in the safe range, ie: it is correctly represented by JavaScript (where all numbers, including integers, are technically floating point numbers).

ECMAScript 6

1
2
Number.isSafeInteger(42) === true
Number.isSafeInteger(90578921689765987123) === false

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
function isSafeInteger (n) {
    return (
           typeof n === 'number'
        && Math.round(n) === n
        && -(Math.pow(2, 53) - 1) <= n
        && n <= (Math.pow(2, 53) - 1)
    );
}
isSafeInteger(42) === true;
isSafeInteger(9007199254740992) === false;

Number Comparison

Availability of a standard Epsilon value for more precise comparison of floating point numbers.

ECMAScript 6

1
2
console.log(0.1 + 0.2 === 0.3) // false
console.log(Math.abs((0.1 + 0.2) - 0.3) < Number.EPSILON) // true

ECMAScript 5

1
2
console.log(0.1 + 0.2 === 0.3); // false
console.log(Math.abs((0.1 + 0.2) - 0.3) < 2.220446049250313e-16); // true

Number Truncation

Truncate a floating point number to its integral part, completely dropping the fractional part.

ECMAScript 6

1
2
3
console.log(Math.trunc(42.7)) // 42
console.log(Math.trunc(0.1)) // 0
console.log(Math.trunc(-0.1)) // -0

ECMAScript 5

1
2
3
4
5
6
function mathTrunc (x) {
    return (x < 0 ? Math.ceil(x) : Math.floor(x));
}
console.log(mathTrunc(42.7)) // 42
console.log(mathTrunc( 0.1)) // 0
console.log(mathTrunc(-0.1)) // -0

Number Sign Determination

Determine the sign of a number, including special cases of signed zero and non-number.

ECMAScript 6

1
2
3
4
5
console.log(Math.sign(7))   // 1
console.log(Math.sign(0))   // 0
console.log(Math.sign(-0))  // -0
console.log(Math.sign(-7))  // -1
console.log(Math.sign(NaN)) // NaN

ECMAScript 5

1
2
3
4
5
6
7
8
function mathSign (x) {
    return ((x === 0 || isNaN(x)) ? x : (x > 0 ? 1 : -1));
}
console.log(mathSign(7))   // 1
console.log(mathSign(0))   // 0
console.log(mathSign(-0))  // -0
console.log(mathSign(-7))  // -1
console.log(mathSign(NaN)) // NaN

Promises

Promise Usage

First class representation of a value that may be made asynchronously and be available in the future.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
function msgAfterTimeout (msg, who, timeout) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(`${msg} Hello ${who}!`), timeout)
    })
}
msgAfterTimeout("", "Foo", 100).then((msg) =>
    msgAfterTimeout(msg, "Bar", 200)
).then((msg) => {
    console.log(`done after 300ms:${msg}`)
})

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
function msgAfterTimeout (msg, who, timeout, onDone) {
    setTimeout(function () {
        onDone(msg + " Hello " + who + "!");
    }, timeout);
}
msgAfterTimeout("", "Foo", 100, function (msg) {
    msgAfterTimeout(msg, "Bar", 200, function (msg) {
        console.log("done after 300ms:" + msg);
    });
});

Promise Combination

Combine one or more promises into new promises without having to take care of ordering of the underlying asynchronous operations yourself.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
function fetchAsync (url, timeout, onData, onError) {
    ...
}
let fetchPromised = (url, timeout) => {
    return new Promise((resolve, reject) => {
        fetchAsync(url, timeout, resolve, reject)
    })
}
Promise.all([
    fetchPromised("http://backend/foo.txt", 500),
    fetchPromised("http://backend/bar.txt", 500),
    fetchPromised("http://backend/baz.txt", 500)
]).then((data) => {
    let [ foo, bar, baz ] = data
    console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
}, (err) => {
    console.log(`error: ${err}`)
})

ECMAScript 5

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
function fetchAsync (url, timeout, onData, onError) {
    …
}
function fetchAll (request, onData, onError) {
    var result = [], results = 0;
    for (var i = 0; i < request.length; i++) {
        result[i] = null;
        (function (i) {
            fetchAsync(request[i].url, request[i].timeout, function (data) {
                result[i] = data;
                if (++results === request.length)
                    onData(result);
            }, onError);
        })(i);
    }
}
fetchAll([
    { url: "http://backend/foo.txt", timeout: 500 },
    { url: "http://backend/bar.txt", timeout: 500 },
    { url: "http://backend/baz.txt", timeout: 500 }
], function (data) {
    var foo = data[0], bar = data[1], baz = data[2];
    console.log("success: foo=" + foo + " bar=" + bar + " baz=" + baz);
}, function (err) {
    console.log("error: " + err);
});

Meta-Programming

Proxying

Hooking into runtime-level object meta-operations.

ECMAScript 6

1
2
3
4
5
6
7
8
9
10
let target = {
    foo: "Welcome, foo"
}
let proxy = new Proxy(target, {
    get (receiver, name) {
        return name in receiver ? receiver[name] : `Hello, ${name}`
    }
})
proxy.foo === "Welcome, foo"
proxy.world === "Hello, world"

ECMAScript 5

1
// no equivalent in ES5

Reflection

Make calls corresponding to the object meta-operations.

ECMAScript 6

1
2
3
4
let obj = { a: 1 }
Object.defineProperty(obj, "b", { value: 2 })
obj[Symbol("c")] = 3
Reflect.ownKeys(obj) // [ "a", "b", Symbol(c) ]

ECMAScript 5

1
2
3
4
var obj = { a: 1 };
Object.defineProperty(obj, "b", { value: 2 });
// no equivalent in ES5
Object.getOwnPropertyNames(obj); // [ "a", "b" ]

Internationalization & Localization

Collation

Sorting a set of strings and searching within a set of strings. Collation is parameterized by locale and aware of Unicode.

ECMAScript 6

1
2
3
4
5
6
7
8
9
// in German,  "ä" sorts with "a"
// in Swedish, "ä" sorts after "z"
var list = [ "ä", "a", "z" ]
var l10nDE = new Intl.Collator("de")
var l10nSV = new Intl.Collator("sv")
l10nDE.compare("ä", "z") === -1
l10nSV.compare("ä", "z") === +1
console.log(list.sort(l10nDE.compare)) // [ "a", "ä", "z" ]
console.log(list.sort(l10nSV.compare)) // [ "a", "z", "ä" ]

ECMAScript 5

1
// no equivalent in ES5

Number Formatting

Format numbers with digit grouping and localized separators.

ECMAScript 6

1
2
3
4
var l10nEN = new Intl.NumberFormat("en-US")
var l10nDE = new Intl.NumberFormat("de-DE")
l10nEN.format(1234567.89) === "1,234,567.89"
l10nDE.format(1234567.89) === "1.234.567,89"

ECMAScript 5

1
// no equivalent in ES5

Currency Formatting

Format numbers with digit grouping, localized separators, and attached currency symbol.

ECMAScript 6

1
2
3
4
5
6
var l10nUSD = new Intl.NumberFormat("en-US", { style: "currency", currency: "USD" })
var l10nGBP = new Intl.NumberFormat("en-GB", { style: "currency", currency: "GBP" })
var l10nEUR = new Intl.NumberFormat("de-DE", { style: "currency", currency: "EUR" })
l10nUSD.format(100200300.40) === "$100,200,300.40"
l10nGBP.format(100200300.40) === "£100,200,300.40"
l10nEUR.format(100200300.40) === "100.200.300,40 €"

ECMAScript 5

1
// no equivalent in ES5

Date/Time Formatting

Format date/time with localized ordering and separators.

ECMAScript 6:

1
2
3
4
var l10nEN = new Intl.DateTimeFormat("en-US")
var l10nDE = new Intl.DateTimeFormat("de-DE")
l10nEN.format(new Date("2015-01-02")) === "1/2/2015"
l10nDE.format(new Date("2015-01-02")) === "2.1.2015"

ECMAScript 5:

1
// no equivalent in ES5