Skip to content

Latest commit

 

History

History
410 lines (358 loc) · 33.7 KB

es6.md

File metadata and controls

410 lines (358 loc) · 33.7 KB
Raw

350 个特性看透 ES6

Introduction

  • ES6 – also known as Harmony, es-next, ES2015 – is the latest finalized specification of the language
  • The ES6 specification was finalized in June 2015, (hence ES2015)
  • Future versions of the specification will follow the ES[YYYY] pattern, e.g ES2016 for ES7
    • Yearly release schedule, features that don’t make the cut take the next train
    • Since ES6 pre-dates that decision, most of us still call it ES6
    • Starting with ES2016 (ES7), we should start using the ES[YYYY] pattern to refer to newer versions
    • Top reason for naming scheme is to pressure browser vendors into quickly implementing newest features

Tooling

  • To get ES6 working today, you need a JavaScript-to-JavaScript transpiler
  • Transpilers are here to stay
    • They allow you to compile code in the latest version into older versions of the language
    • As browser support gets better, we’ll transpile ES2016 and ES2017 into ES6 and beyond
    • We’ll need better source mapping functionality
    • They’re the most reliable way to run ES6 source code in production today (although browsers get ES5)
  • Babel (a transpiler) has a killer feature: human-readable output
  • Use babel to transpile ES6 into ES5 for static builds
  • Use babelify to incorporate babel into your Gulp, Grunt, or npm run build process
  • Use Node.js v4.x.x or greater as they have decent ES6 support baked in, thanks to v8
  • Use babel-node with any version of node, as it transpiles modules into ES5
  • Babel has a thriving ecosystem that already supports some of ES2016 and has plugin support
  • Read A Brief History of ES6 Tooling

Assignment Destructuring

  • var {foo} = pony is equivalent to var foo = pony.foo
  • var {foo: baz} = pony is equivalent to var baz = pony.foo
  • You can provide default values, var {foo='bar'} = baz yields foo: 'bar' if baz.foo is undefined
  • You can pull as many properties as you like, aliased or not
    • var {foo, bar: baz} = {foo: 0, bar: 1} gets you foo: 0 and baz: 1
  • You can go deeper. var {foo: {bar}} = { foo: { bar: 'baz' } } gets you bar: 'baz'
  • You can alias that too. var {foo: {bar: deep}} = { foo: { bar: 'baz' } } gets you deep: 'baz'
  • Properties that aren’t found yield undefined as usual, e.g: var {foo} = {}
  • Deeply nested properties that aren’t found yield an error, e.g: var {foo: {bar}} = {}
  • It also works for arrays, [a, b] = [0, 1] yields a: 0 and b: 1
  • You can skip items in an array, [a, , b] = [0, 1, 2], getting a: 0 and b: 2
  • You can swap without an “aux” variable, [a, b] = [b, a]
  • You can also use destructuring in function parameters
    • Assign default values like function foo (bar=2) {}
    • Those defaults can be objects, too function foo (bar={ a: 1, b: 2 }) {}
    • Destructure bar completely, like function foo ({ a=1, b=2 }) {}
    • Default to an empty object if nothing is provided, like function foo ({ a=1, b=2 } = {}) {}
  • Read ES6 JavaScript Destructuring in Depth

Spread Operator and Rest Parameters

  • Rest parameters is a better arguments
    • You declare it in the method signature like function foo (...everything) {}
    • everything is an array with all parameters passed to foo
    • You can name a few parameters before ...everything, like function foo (bar, ...rest) {}
    • Named parameters are excluded from ...rest
    • ...rest must be the last parameter in the list
  • Spread operator is better than magic, also denoted with ... syntax
    • Avoids .apply when calling methods, fn(...[1, 2, 3]) is equivalent to fn(1, 2, 3)
    • Easier concatenation [1, 2, ...[3, 4, 5], 6, 7]
    • Casts array-likes or iterables into an array, e.g [...document.querySelectorAll('img')]
    • Useful when destructuring too, [a, , ...rest] = [1, 2, 3, 4, 5] yields a: 1 and rest: [3, 4, 5]
    • Makes new + .apply effortless, new Date(...[2015, 31, 8])
  • Read ES6 Spread and Butter in Depth

Arrow Functions

  • Terse way to declare a function like param => returnValue
  • Useful when doing functional stuff like [1, 2].map(x => x * 2)
  • Several flavors are available, might take you some getting used to
    • p1 => expr is okay for a single parameter
    • p1 => expr has an implicit return statement for the provided expr expression
    • To return an object implicitly, wrap it in parenthesis () => ({ foo: 'bar' }) or you’ll get an error
    • Parenthesis are demanded when you have zero, two, or more parameters, () => expr or (p1, p2) => expr
    • Brackets in the right-hand side represent a code block that can have multiple statements, () => {}
    • When using a code block, there’s no implicit return, you’ll have to provide it – () => { return 'foo' }
  • You can’t name arrow functions statically, but runtimes are now much better at inferring names for most methods
  • Arrow functions are bound to their lexical scope
    • this is the same this context as in the parent scope
    • this can’t be modified with .call, .apply, or similar “reflection”-type methods
  • Read ES6 Arrow Functions in Depth

Template Literals

  • You can declare strings with ``` (backticks), in addition to " and `'`
  • Strings wrapped in backticks are template literals
  • Template literals can be multiline
  • Template literals allow interpolation like ponyfoo.com is ${rating} where rating is a variable
  • You can use any valid JavaScript expressions in the interpolation, such as ${2 * 3} or ${foo()}
  • You can use tagged templates to change how expressions are interpolated
    • Add a fn prefix to fnfoo, ${bar} and ${baz}``
    • fn is called once with template, ...expressions
    • template is ['foo, ', ' and ', ''] and expressions is [bar, baz]
    • The result of fn becomes the value of the template literal
    • Possible use cases include input sanitization of expressions, parameter parsing, etc.
  • Template literals are almost strictly better than strings wrapped in single or double quotes
  • Read ES6 Template Literals in Depth

Object Literals

  • Instead of { foo: foo }, you can just do { foo } – known as a property value shorthand
  • Computed property names, { [prefix + 'Foo']: 'bar' }, where prefix: 'moz', yields { mozFoo: 'bar' }
  • You can’t combine computed property names and property value shorthands, { [foo] } is invalid
  • Method definitions in an object literal can be declared using an alternative, more terse syntax, { foo () {} }
  • See also Object section
  • Read ES6 Object Literal Features in Depth

Classes

  • Not “traditional” classes, syntax sugar on top of prototypal inheritance
  • Syntax similar to declaring objects, class Foo {}
  • Instance methods – new Foo().bar – are declared using the short object literal syntax, class Foo { bar () {} }
  • Static methods – Foo.isPonyFoo() – need a static keyword prefix, class Foo { static isPonyFoo () {} }
  • Constructor method class Foo { constructor () { /* initialize instance */ } }
  • Prototypal inheritance with a simple syntax class PonyFoo extends Foo {}
  • Read ES6 Classes in Depth

Let and Const

  • let and const are alternatives to var when declaring variables
  • let is block-scoped instead of lexically scoped to a function
  • let is hoisted to the top of the block, while var declarations are hoisted to top of the function
  • “Temporal Dead Zone” – TDZ for short
    • Starts at the beginning of the block where let foo was declared
    • Ends where the let foo statement was placed in user code (hoisiting is irrelevant here)
    • Attempts to access or assign to foo within the TDZ (before the let foo statement is reached) result in an error
    • Helps prevent mysterious bugs when a variable is manipulated before its declaration is reached
  • const is also block-scoped, hoisted, and constrained by TDZ semantics
  • const variables must be declared using an initializer, const foo = 'bar'
  • Assigning to const after initialization fails silently (or loudly – with an exception – under strict mode)
  • const variables don’t make the assigned value immutable
    • const foo = { bar: 'baz' } means foo will always reference the right-hand side object
    • const foo = { bar: 'baz' }; foo.bar = 'boo' won’t throw
  • Declaration of a variable by the same name will throw
  • Meant to fix mistakes where you reassign a variable and lose a reference that was passed along somewhere else
  • In ES6, functions are block scoped
    • Prevents leaking block-scoped secrets through hoisting, { let _foo = 'secret', bar = () => _foo; }
    • Doesn’t break user code in most situations, and typically what you wanted anyways
  • Read ES6 Let, Const and the “Temporal Dead Zone” (TDZ) in Depth

Symbols

  • A new primitive type in ES6
  • You can create your own symbols using var symbol = Symbol()
  • You can add a description for debugging purposes, like Symbol('ponyfoo')
  • Symbols are immutable and unique. Symbol(), Symbol(), Symbol('foo') and Symbol('foo') are all different
  • Symbols are of type symbol, thus: typeof Symbol() === 'symbol'
  • You can also create global symbols with Symbol.for(key)
    • If a symbol with the provided key already existed, you get that one back
    • Otherwise, a new symbol is created, using key as its description as well
    • Symbol.keyFor(symbol) is the inverse function, taking a symbol and returning its key
    • Global symbols are as global as it gets, or cross-realm. Single registry used to look up these symbols across the runtime
      • window context
      • eval context
      • <iframe>context, Symbol.for('foo') === iframe.contentWindow.Symbol.for('foo')
  • There’s also “well-known” symbols
    • Not on the global registry, accessible through Symbol[name], e.g: Symbol.iterator
    • Cross-realm, meaning Symbol.iterator === iframe.contentWindow.Symbol.iterator
    • Used by specification to define protocols, such as the iterable protocol over Symbol.iterator
    • They’re not actually well-known – in colloquial terms
  • Iterating over symbol properties is hard, but not impossible and definitely not private
    • Symbols are hidden to all pre-ES6 “reflection” methods
    • Symbols are accessible through Object.getOwnPropertySymbols
    • You won’t stumble upon them but you will find them if actively looking
  • Read ES6 Symbols in Depth

Iterators

  • Iterator and iterable protocol define how to iterate over any object, not just arrays and array-likes
  • A well-known Symbol is used to assign an iterator to any object
  • var foo = { [Symbol.iterator]: iterable}, or foo[Symbol.iterator] = iterable
  • The iterable is a method that returns an iterator object that has a next method
  • The next method returns objects with two properties, value and done
    • The value property indicates the current value in the sequence being iterated
    • The done property indicates whether there are any more items to iterate
  • Objects that have a [Symbol.iterator] value are iterable, because they subscribe to the iterable protocol
  • Some built-ins like Array, String, or arguments – and NodeList in browsers – are iterable by default in ES6
  • Iterable objects can be looped over with for..of, such as for (let el of document.querySelectorAll('a'))
  • Iterable objects can be synthesized using the spread operator, like [...document.querySelectorAll('a')]
  • You can also use Array.from(document.querySelectorAll('a')) to synthesize an iterable sequence into an array
  • Iterators are lazy, and those that produce an infinite sequence still can lead to valid programs
  • Be careful not to attempt to synthesize an infinite sequence with ... or Array.from as that will cause an infinite loop
  • Read ES6 Iterators in Depth

Generators

  • Generator functions are a special kind of iterator that can be declared using the function* generator () {} syntax
  • Generator functions use yield to emit an element sequence
  • Generator functions can also use yield* to delegate to another generator function – or any iterable object
  • Generator functions return a generator object that’s adheres to both the iterable and iterator protocols
    • Given g = generator(), g adheres to the iterable protocol because g[Symbol.iterator] is a method
    • Given g = generator(), g adheres to the iterator protocol because g.next is a method
    • The iterator for a generator object g is the generator itself: g[Symbol.iterator]() === g
  • Pull values using Array.from(g), [...g], for (let item of g), or just calling g.next()
  • Generator function execution is suspended, remembering the last position, in four different cases
    • A yield expression returning the next value in the sequence
    • A return statement returning the last value in the sequence
    • A throw statement halts execution in the generator entirely
    • Reaching the end of the generator function signals { done: true }
  • Once the g sequence has ended, g.next() simply returns { done: true } and has no effect
  • It’s easy to make asynchronous flows feel synchronous
    • Take user-provided generator function
    • User code is suspended while asynchronous operations take place
    • Call g.next(), unsuspending execution in user code
  • Read ES6 Generators in Depth

Promises

  • Follows the Promises/A+ specification, was widely implemented in the wild before ES6 was standarized (e.g bluebird)
  • Promises behave like a tree. Add branches with p.then(handler) and p.catch(handler)
  • Create new p promises with new Promise((resolve, reject) => { /* resolver */ })
    • The resolve(value) callback will fulfill the promise with the provided value
    • The reject(reason) callback will reject p with a reason error
    • You can call those methods asynchronously, blocking deeper branches of the promise tree
  • Each call to p.then and p.catch creates another promise that’s blocked on p being settled
  • Promises start out in pending state and are settled when they’re either fulfilled or rejected
  • Promises can only be settled once, and then they’re settled. Settled promises unblock deeper branches
  • You can tack as many promises as you want onto as many branches as you need
  • Each branch will execute either .then handlers or .catch handlers, never both
  • A .then callback can transform the result of the previous branch by returning a value
  • A .then callback can block on another promise by returning it
  • p.catch(fn).catch(fn) won’t do what you want – unless what you wanted is to catch errors in the error handler
  • Promise.resolve(value) creates a promise that’s fulfilled with the provided value
  • Promise.reject(reason) creates a promise that’s rejected with the provided reason
  • Promise.all(...promises) creates a promise that settles when all ...promises are fulfilled or 1 of them is rejected
  • Promise.race(...promises) creates a promise that settles as soon as 1 of ...promises is settled
  • Use Promisees – the promise visualization playground – to better understand promises
  • Read ES6 Promises in Depth

Maps

  • A replacement to the common pattern of creating a hash-map using plain JavaScript objects
    • Avoids security issues with user-provided keys
    • Allows keys to be arbitrary values, you can even use DOM elements or functions as the key to an entry
  • Map adheres to iterable protocol
  • Create a map using new Map()
  • Initialize a map with an iterable like [[key1, value1], [key2, value2]] in new Map(iterable)
  • Use map.set(key, value) to add entries
  • Use map.get(key) to get an entry
  • Check for a key using map.has(key)
  • Remove entries with map.delete(key)
  • Iterate over map with for (let [key, value] of map), the spread operator, Array.from, etc
  • Read ES6 Maps in Depth

WeakMaps

  • Similar to Map, but not quite the same
  • WeakMap isn’t iterable, so you don’t get enumeration methods like .forEach, .clear, and others you had in Map
  • WeakMap keys must be reference types. You can’t use value types like symbols, numbers, or strings as keys
  • WeakMap entries with a key that’s the only reference to the referenced variable are subject to garbage collection
  • That last point means WeakMap is great at keeping around metadata for objects, while those objects are still in use
  • You avoid memory leaks, without manual reference counting – think of WeakMap as IDisposable in .NET
  • Read ES6 WeakMaps in Depth

Sets

  • Similar to Map, but not quite the same
  • Set doesn’t have keys, there’s only values
  • set.set(value) doesn’t look right, so we have set.add(value) instead
  • Sets can’t have duplicate values because the values are also used as keys
  • Read ES6 Sets in Depth

WeakSets

  • WeakSet is sort of a cross-breed between Set and WeakMap
  • A WeakSet is a set that can’t be iterated and doesn’t have enumeration methods
  • WeakSet values must be reference types
  • WeakSet may be useful for a metadata table indicating whether a reference is actively in use or not
  • Read ES6 WeakSets in Depth

Proxies

  • Proxies are created with new Proxy(target, handler), where target is any object and handler is configuration
  • The default behavior of a proxy acts as a passthrough to the underlying target object
  • Handlers determine how the underlying target object is accessed on top of regular object property access semantics
  • You pass off references to proxy and retain strict control over how target can be interacted with
  • Handlers are also known as traps, these terms are used interchangeably
  • You can create revocable proxies with Proxy.revocable(target, handler)
    • That method returns an object with proxy and revoke properties
    • You could destructure var {proxy, revoke} = Proxy.revocable(target, handler) for convenience
    • You can configure the proxy all the same as with new Proxy(target, handler)
    • After revoke() is called, the proxy will throw on any operation, making it convenient when you can’t trust consumers
  • get – traps proxy.prop and proxy['prop']
  • set – traps proxy.prop = value and proxy['prop'] = value
  • has – traps in operator
  • deleteProperty – traps delete operator
  • defineProperty – traps Object.defineProperty and declarative alternatives
  • enumerate – traps for..in loops
  • ownKeys – traps Object.keys and related methods
  • apply – traps function calls
  • construct – traps usage of the new operator
  • getPrototypeOf – traps internal calls to [[GetPrototypeOf]]
  • setPrototypeOf – traps calls to Object.setPrototypeOf
  • isExtensible – traps calls to Object.isExtensible
  • preventExtensions – traps calls to Object.preventExtensions
  • getOwnPropertyDescriptor – traps calls to Object.getOwnPropertyDescriptor
  • Read ES6 Proxies in Depth
  • Read ES6 Proxy Traps in Depth
  • Read More ES6 Proxy Traps in Depth

Reflection

  • Reflection is a new static built-in (think of Math) in ES6
  • Reflection methods have sensible internals, e.g Reflect.defineProperty returns a boolean instead of throwing
  • There’s a Reflection method for each proxy trap handler, and they represent the default behavior of each trap
  • Going forward, new reflection methods in the same vein as Object.keys will be placed in the Reflection namespace
  • Read ES6 Reflection in Depth

Number

  • Use 0b prefix for binary, and 0o prefix for octal integer literals
  • Number.isNaN and Number.isFinite are like their global namesakes, except that they don’t coerce input to Number
  • Number.parseInt and Number.parseFloat are exactly the same as their global namesakes
  • Number.isInteger checks if input is a Number value that doesn’t have a decimal part
  • Number.EPSILON helps figure out negligible differences between two numbers – e.g. 0.1 + 0.2 and 0.3
  • Number.MAX_SAFE_INTEGER is the largest integer that can be safely and precisely represented in JavaScript
  • Number.MIN_SAFE_INTEGER is the smallest integer that can be safely and precisely represented in JavaScript
  • Number.isSafeInteger checks whether an integer is within those bounds, able to be represented safely and precisely
  • Read ES6 Number Improvements in Depth

Math

Array

Object

Strings and Unicode

Modules

  • Strict Mode is turned on by default in the ES6 module system
  • ES6 modules are files that export an API
  • export default value exports a default binding
  • export var foo = 'bar' exports a named binding
  • Named exports are bindings that can be changed at any time from the module that’s exporting them
  • export { foo, bar } exports a list of named exports
  • export { foo as ponyfoo } aliases the export to be referenced as ponyfoo instead
  • export { foo as default } marks the named export as the default export
  • As a best practice, export default api at the end of all your modules, where api is an object, avoids confusion
  • Module loading is implementation-specific, allows interoperation with CommonJS
  • import 'foo' loads the foo module into the current module
  • import foo from 'ponyfoo' assigns the default export of ponyfoo to a local foo variable
  • import {foo, bar} from 'baz' imports named exports foo and bar from the baz module
  • import {foo as bar} from 'baz' imports named export foo but aliased as a bar variable
  • import {default} from 'foo' also imports the default export
  • import {default as bar} from 'foo' imports the default export aliased as bar
  • import foo, {bar, baz} from 'foo' mixes default foo with named exports bar and baz in one declaration
  • import * as foo from 'foo' imports the namespace object
    • Contains all named exports in foo[name]
    • Contains the default export in foo.default, if a default export was declared in the module
  • Read ES6 Modules Additions in Depth

Time for a bullet point detox. Then again, I did warn you to read the article series instead. Don’t forget to subscribe and maybe even contribute to keep Pony Foo alive. Also, did you try the Konami code just yet?