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ECMAScript 6 – a tour
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Ankur Sethi
April 25, 2015
Programming
1
220
ECMAScript 6 – a tour
A short tour of some of the new features in ECMAScript 6. Presented at BangaloreJS.
Ankur Sethi
April 25, 2015
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Transcript
ECMAScript 6 A tour
The ‘let’ statement
var myvar = 'global'; function f() { console.log(myvar); if (true)
{ var myvar = 'local'; } console.log(myvar); }
var myvar = 'global'; function f() { console.log(myvar); if (true)
{ var myvar = 'local'; } console.log(myvar); } > undefined > local
Hoisting
function infiltrate() { // do something // ... var bulletCount
= 10; // do something else // ... var handler = function() { // handle difficult things }; }
function infiltrate() { var bulletCount; var handler; // do something
// ... bulletCount = 10; // do something else // ... handler = function() { // handle difficult things }; }
function f() { console.log(bulletCount); var bulletCount = 42; console.log(bulletCount); }
function f() { console.log(bulletCount); var bulletCount = 42; console.log(bulletCount); }
> undefined > 42
'use strict'; function f() { console.log(bulletCount); let bulletCount = 42;
console.log(bulletCount); } > ReferenceError: can't access lexical declaration `bulletCount’ before initialization
Function Scope
function getAnswer() { if (true) { var theAnswer = 42;
} console.log(theAnswer); }
function getAnswer() { if (true) { var theAnswer = 42;
} console.log(theAnswer); } > 42
'use strict'; function getAnswer() { if (true) { let theAnswer
= 42; } console.log(theAnswer); } > ReferenceError: theAnswer is not defined
‘use strict’; let myvar = 'global'; function f() { console.log(myvar);
if (true) { let myvar = 'local'; } console.log(myvar); }
‘use strict’; let myvar = 'global'; function f() { console.log(myvar);
if (true) { let myvar = 'local'; } console.log(myvar); } > global > global
‘use strict’; let myvar = 'global'; function f() { console.log(myvar);
if (true) { let myvar = ‘local'; console.log(myvar); } }
‘use strict’; let myvar = 'global'; function f() { console.log(myvar);
if (true) { let myvar = ‘local'; console.log(myvar); } } > global > local
Protip #1 Use ‘let’ for mutable references
The ‘const’ statement
'use strict'; const theAnswer = 42; theAnswer = 87;
'use strict'; const theAnswer = 42; theAnswer = 87; >
Exception: SyntaxError: invalid assignment to const foo
'use strict'; const agents = { 'Archer': 'Sterling', 'Kane': 'Lana'
}; agents['Figgis'] = 'Cyril';
'use strict'; const agents = { 'Archer': 'Sterling', 'Kane': 'Lana'
}; agents = { 'Figgis': 'Cyril' };
'use strict'; const agents = { 'Archer': 'Sterling', 'Kane': 'Lana'
}; agents = { 'Figgis': 'Cyril' }; > Exception: SyntaxError: invalid assignment to const agents
Protip #2 Use ‘const’ for immutable references
Protip #3 Replace all usages of ‘var’ with either ‘let’
or ‘const’ depending on context
The array spread operator
var theNumbers = [4, 8, 15]; function addThree(a, b, c)
{ return a + b + c; }
var theNumbers = [4, 8, 15]; function addThree(a, b, c)
{ return a + b + c; } // ECMAScript 5 addThree.apply(null, theNumbers);
var theNumbers = [4, 8, 15]; function addThree(a, b, c)
{ return a + b + c; } // ECMAScript 5 addThree.apply(null, theNumbers); // ECMAScript 6 addThree(...theNumbers);
addThree(...theNumbers); addThree(4, 8, 15);
var foo = [15, 16]; var theNumbers = [4, 8,
...foo, 23, 42]; console.log(theNumbers);
> Array [ 4, 8, 15, 16, 23, 42 ]
var foo = [15, 16]; var theNumbers = [4, 8, ...foo, 23, 42]; console.log(theNumbers);
Protip #4 Replace Function.apply with the array spread operator
var theNumbers = [4, 8, 15, 16, 23, 42]; var
theNumbersCopy = [...theNumbers]; Protip #5 Use the array spread operator to copy arrays
Template Strings
let thing = 'piggies'; let quantity = 3; console.log(`${quantity} little
${thing}`); > 3 little piggies
function sayHello(name) { return `Hello, ${name}!`; } console.log(sayHello('Kreiger'));
function sayHello(name) { return `Hello, ${name}!`; } console.log(sayHello('Kreiger')); > Hello,
Kreiger!
let multiline = `This is a multiline string!`;
for…of loops
var theNumbers = [4, 8, 15, 16, 23, 42]; var
total = 0; for (var i in theNumbers) { total += i; }
var theNumbers = [4, 8, 15, 16, 23, 42]; var
total = 0; for (var i in theNumbers) { total += i; } > “0012345”
Array.prototype.sumOfSquares = function() { // Blazing fast sum of squares
code. }; var theNumbers = [4, 8, 15]; for (var i in theNumbers) { console.log(i); }
Array.prototype.sumOfSquares = function() { // Blazing fast sum of squares
code. }; var theNumbers = [4, 8, 15]; for (var i in theNumbers) { console.log(i); } > 0 > 1 > 2 > sumOfSquares
var theNumbers = [4, 8, 15, 16, 23, 42]; var
total = 0; for (var i of theNumbers) { total += i; }
var theNumbers = [4, 8, 15, 16, 23, 42]; var
total = 0; for (var i of theNumbers) { total += i; } > 108
Arrow Functions
var nums = [4, 8, 15, 16, 23, 42]; //
ECMAScript 5 var squares = nums.map(function(n) { return n * n; }); // ECMAScript 6 let squares = nums.map((n) => n * n);
const squareNumber = n => n * n; const squareNumber
= (n) => n * n; const addNumbers = a, b => a + b; // Error! const addNumbers = (a, b) => a + b; const addNumbers = (a, b) => { return a + b; };
function Person() { this.age = 0; setInterval(function growUp() { this.age++;
}, 1000); } var p = new Person();
function Person() { var self = this; self.age = 0;
setInterval(function growUp() { self.age++; }, 1000); } var p = new Person();
function Person() { this.age = 0; setInterval(() => { this.age++;
}, 1000); } var p = new Person();
Computed Object Properties
var p1 = new Person('Sterling', 'Archer'); var p2 = new
Person('Lana', 'Kane'); var ages = {}; ages[p1.firstName + ' ' + p1.lastName] = 37; ages[p1.firstName + ' ' + p1.lastName] = 33; { 'Sterling Archer': 37, 'Lana Kane': 33 }
let p1 = new Person('Sterling', 'Archer'); let p2 = new
Person('Lana', 'Kane'); let ages = { [p1.firstName + ' ' + p1.lastName]: 37, [p2.firstName + ' ' + p2.lastName]: 33 }; { 'Sterling Archer': 37, 'Lana Kane': 33 }
Property Value Shorthand in Object Literals
var agentName = 'Sterling Archer'; var obj = { agentName:
agentName }; { agentName: 'Sterling Archer' }
const agentName = 'Sterling Archer'; const obj = { agentName
}; { agentName: 'Sterling Archer' }
Method Shorthand in Object Literals
var atom = { value: 1, addValue: function (value) {
return atom.value + value; } };
const atom = { value: 1, addValue(value) { return atom.value
+ value; } };
Rest Parameters
function handleEvent(event) { var args = Array.slice.call(null, arguments, 1); args.forEach(function(arg)
{ // do something }); }; handleEvent('click', arg1, arg2, arg3);
function handleEvent(event, ...args) { args.forEach(function(arg) { // do something });
}; handleEvent('click', arg1, arg2, arg3);
Default Parameters
function frobulate(arg1, arg2) { arg1 = arg1 || 23; arg2
= arg2 || 42; } frobulate(); frobulate(10); frobulate(10, 20);
function frobulate(arg1, arg2) { if (typeof arg1 === 'undefined') {
arg1 = 23; } if (typeof arg2 === 'undefined') { arg2 = 42; } } frobulate(); frobulate(10); frobulate(10, 20);
function frobulate(arg1 = 23, arg2 = 42) { // do
something } frobulate(); frobulate(10); frobulate(10, 20);
function frobulate(arg1, arg2 = 42) { // do something }
frobulate(); frobulate(10); frobulate(10, 20);
More! • Classes • Destructuring • Generators • New data
structures (Map, WeakMap, Set, WeakSet) • Promises • Symbols • New methods on Object, String, Array, Math, Number • Better Unicode support • Modules
Use it Now Use shipping features in modern browsers https://kangax.github.io/compat-table/es6/
Use it Now Use io.js https://iojs.org
Use it Now Compile down to ECMAScript 5 with Babel
http://babeljs.io
[email protected]