[javascript pt-br] : first part

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+---
+language: javascript
+contributors:
+    - ["Adam Brenecki", "http://adam.brenecki.id.au"]
+    - ["Ariel Krakowski", "http://www.learneroo.com"]
+filename: javascript.js
+---
+
+JavaScript foi criada por Brendan Eich, funcionário da Netscape, em 1995. Ela
+foi originalmente criada para ser uma linguagem de script para websites,
+complementando o uso de Java para aplicações web mais complexas, mas a sua
+integração com páginas web e seu suporte nativo nos browsers fez com que
+ela se tornasse mais comum que Java no frontend web.
+
+Javascript não é somente limitado a browsers web, no entanto: existe o Node.js,
+que é um projeto que fornece um interpretador baseado no motor V8 do Google 
+Chrome e está se tornando cada vez mais famoso.
+
+
+Feedback são muito apreciados! Você me encontrar em
+[@adambrenecki](https://twitter.com/adambrenecki), ou
+[adam@brenecki.id.au](mailto:adam@brenecki.id.au).
+
+```js
+// Comentários são como em C. Comentários de uma linha começam com duas barras,
+/* e comentários de múltplas linhas começam com barra-asterisco
+   e fecham com asterisco-barra */
+
+// comandos podem ser terminados com  ;
+facaAlgo();
+
+// ... mas eles não precisam ser, assim como o ponto-e-vírgula é automaticamente
+// inserido quando há uma nova linha, exceto alguns casos.
+facaAlgo()
+
+// Porque esses casos podem causar resultados inesperados, vamos continuar 
+// a usar ponto-e-vírgula neste guia.
+
+///////////////////////////////////
+// 1. Numbers, Strings and Operators
+
+// JavaScript has one number type (which is a 64-bit IEEE 754 double).
+// Doubles have a 52-bit mantissa, which is enough to store integers
+//    up to about 9✕10¹⁵ precisely.
+3; // = 3
+1.5; // = 1.5
+
+// Some basic arithmetic works as you'd expect.
+1 + 1; // = 2
+0.1 + 0.2; // = 0.30000000000000004
+8 - 1; // = 7
+10 * 2; // = 20
+35 / 5; // = 7
+
+// Including uneven division.
+5 / 2; // = 2.5
+
+// Bitwise operations also work; when you perform a bitwise operation your float
+// is converted to a signed int *up to* 32 bits.
+1 << 2; // = 4
+
+// Precedence is enforced with parentheses.
+(1 + 3) * 2; // = 8
+
+// There are three special not-a-real-number values:
+Infinity; // result of e.g. 1/0
+-Infinity; // result of e.g. -1/0
+NaN; // result of e.g. 0/0
+
+// There's also a boolean type.
+true;
+false;
+
+// Strings are created with ' or ".
+'abc';
+"Hello, world";
+
+// Negation uses the ! symbol
+!true; // = false
+!false; // = true
+
+// Equality is ===
+1 === 1; // = true
+2 === 1; // = false
+
+// Inequality is !==
+1 !== 1; // = false
+2 !== 1; // = true
+
+// More comparisons
+1 < 10; // = true
+1 > 10; // = false
+2 <= 2; // = true
+2 >= 2; // = true
+
+// Strings are concatenated with +
+"Hello " + "world!"; // = "Hello world!"
+
+// and are compared with < and >
+"a" < "b"; // = true
+
+// Type coercion is performed for comparisons with double equals...
+"5" == 5; // = true
+null == undefined; // = true
+
+// ...unless you use ===
+"5" === 5; // = false
+null === undefined; // = false 
+
+// ...which can result in some weird behaviour...
+13 + !0; // 14
+"13" + !0; // '13true'
+
+// You can access characters in a string with `charAt`
+"This is a string".charAt(0);  // = 'T'
+
+// ...or use `substring` to get larger pieces.
+"Hello world".substring(0, 5); // = "Hello"
+
+// `length` is a property, so don't use ().
+"Hello".length; // = 5
+
+// There's also `null` and `undefined`.
+null;      // used to indicate a deliberate non-value
+undefined; // used to indicate a value is not currently present (although
+           // `undefined` is actually a value itself)
+
+// false, null, undefined, NaN, 0 and "" are falsy; everything else is truthy.
+// Note that 0 is falsy and "0" is truthy, even though 0 == "0".
+
+///////////////////////////////////
+// 2. Variables, Arrays and Objects
+
+// Variables are declared with the `var` keyword. JavaScript is dynamically
+// typed, so you don't need to specify type. Assignment uses a single `=`
+// character.
+var someVar = 5;
+
+// if you leave the var keyword off, you won't get an error...
+someOtherVar = 10;
+
+// ...but your variable will be created in the global scope, not in the scope
+// you defined it in.
+
+// Variables declared without being assigned to are set to undefined.
+var someThirdVar; // = undefined
+
+// There's shorthand for performing math operations on variables:
+someVar += 5; // equivalent to someVar = someVar + 5; someVar is 10 now
+someVar *= 10; // now someVar is 100
+
+// and an even-shorter-hand for adding or subtracting 1
+someVar++; // now someVar is 101
+someVar--; // back to 100
+
+// Arrays are ordered lists of values, of any type.
+var myArray = ["Hello", 45, true];
+
+// Their members can be accessed using the square-brackets subscript syntax.
+// Array indices start at zero.
+myArray[1]; // = 45
+
+// Arrays are mutable and of variable length.
+myArray.push("World");
+myArray.length; // = 4
+
+// Add/Modify at specific index
+myArray[3] = "Hello";
+
+// JavaScript's objects are equivalent to "dictionaries" or "maps" in other
+// languages: an unordered collection of key-value pairs.
+var myObj = {key1: "Hello", key2: "World"};
+
+// Keys are strings, but quotes aren't required if they're a valid
+// JavaScript identifier. Values can be any type.
+var myObj = {myKey: "myValue", "my other key": 4};
+
+// Object attributes can also be accessed using the subscript syntax,
+myObj["my other key"]; // = 4
+
+// ... or using the dot syntax, provided the key is a valid identifier.
+myObj.myKey; // = "myValue"
+
+// Objects are mutable; values can be changed and new keys added.
+myObj.myThirdKey = true;
+
+// If you try to access a value that's not yet set, you'll get undefined.
+myObj.myFourthKey; // = undefined
+
+///////////////////////////////////
+// 3. Logic and Control Structures
+
+// The syntax for this section is almost identical to Java's. 
+
+// The `if` structure works as you'd expect.
+var count = 1;
+if (count == 3){
+    // evaluated if count is 3
+} else if (count == 4){
+    // evaluated if count is 4
+} else {
+    // evaluated if it's not either 3 or 4
+}
+
+// As does `while`.
+while (true){
+    // An infinite loop!
+}
+
+// Do-while loops are like while loops, except they always run at least once.
+var input;
+do {
+    input = getInput();
+} while (!isValid(input))
+
+// The `for` loop is the same as C and Java:
+// initialisation; continue condition; iteration.
+for (var i = 0; i < 5; i++){
+    // will run 5 times
+}
+
+// && is logical and, || is logical or
+if (house.size == "big" && house.colour == "blue"){
+    house.contains = "bear";
+}
+if (colour == "red" || colour == "blue"){
+    // colour is either red or blue
+}
+
+// && and || "short circuit", which is useful for setting default values.
+var name = otherName || "default";
+
+
+// The `switch` statement checks for equality with `===`.
+// use 'break' after each case 
+// or the cases after the correct one will be executed too. 
+grade = 'B';
+switch (grade) {
+  case 'A':
+    console.log("Great job");
+    break;
+  case 'B':
+    console.log("OK job");
+    break;
+  case 'C':
+    console.log("You can do better");
+    break;
+  default:
+    console.log("Oy vey");
+    break;
+}
+
+
+///////////////////////////////////
+// 4. Functions, Scope and Closures
+
+// JavaScript functions are declared with the `function` keyword.
+function myFunction(thing){
+    return thing.toUpperCase();
+}
+myFunction("foo"); // = "FOO"
+
+// Note that the value to be returned must start on the same line as the
+// `return` keyword, otherwise you'll always return `undefined` due to
+// automatic semicolon insertion. Watch out for this when using Allman style.
+function myFunction()
+{
+    return // <- semicolon automatically inserted here
+    {
+        thisIsAn: 'object literal'
+    }
+}
+myFunction(); // = undefined
+
+// JavaScript functions are first class objects, so they can be reassigned to
+// different variable names and passed to other functions as arguments - for
+// example, when supplying an event handler:
+function myFunction(){
+    // this code will be called in 5 seconds' time
+}
+setTimeout(myFunction, 5000);
+// Note: setTimeout isn't part of the JS language, but is provided by browsers
+// and Node.js.
+
+// Function objects don't even have to be declared with a name - you can write
+// an anonymous function definition directly into the arguments of another.
+setTimeout(function(){
+    // this code will be called in 5 seconds' time
+}, 5000);
+
+// JavaScript has function scope; functions get their own scope but other blocks
+// do not.
+if (true){
+    var i = 5;
+}
+i; // = 5 - not undefined as you'd expect in a block-scoped language
+
+// This has led to a common pattern of "immediately-executing anonymous
+// functions", which prevent temporary variables from leaking into the global
+// scope.
+(function(){
+    var temporary = 5;
+    // We can access the global scope by assiging to the "global object", which
+    // in a web browser is always `window`. The global object may have a
+    // different name in non-browser environments such as Node.js.
+    window.permanent = 10;
+})();
+temporary; // raises ReferenceError
+permanent; // = 10
+
+// One of JavaScript's most powerful features is closures. If a function is
+// defined inside another function, the inner function has access to all the
+// outer function's variables, even after the outer function exits.
+function sayHelloInFiveSeconds(name){
+    var prompt = "Hello, " + name + "!";
+    // Inner functions are put in the local scope by default, as if they were
+    // declared with `var`.
+    function inner(){
+        alert(prompt);
+    }
+    setTimeout(inner, 5000);
+    // setTimeout is asynchronous, so the sayHelloInFiveSeconds function will
+    // exit immediately, and setTimeout will call inner afterwards. However,
+    // because inner is "closed over" sayHelloInFiveSeconds, inner still has
+    // access to the `prompt` variable when it is finally called.
+}
+sayHelloInFiveSeconds("Adam"); // will open a popup with "Hello, Adam!" in 5s
+
+///////////////////////////////////
+// 5. More about Objects; Constructors and Prototypes
+
+// Objects can contain functions.
+var myObj = {
+    myFunc: function(){
+        return "Hello world!";
+    }
+};
+myObj.myFunc(); // = "Hello world!"
+
+// When functions attached to an object are called, they can access the object
+// they're attached to using the `this` keyword.
+myObj = {
+    myString: "Hello world!",
+    myFunc: function(){
+        return this.myString;
+    }
+};
+myObj.myFunc(); // = "Hello world!"
+
+// What this is set to has to do with how the function is called, not where
+// it's defined. So, our function doesn't work if it isn't called in the
+// context of the object.
+var myFunc = myObj.myFunc;
+myFunc(); // = undefined
+
+// Inversely, a function can be assigned to the object and gain access to it
+// through `this`, even if it wasn't attached when it was defined.
+var myOtherFunc = function(){
+    return this.myString.toUpperCase();
+}
+myObj.myOtherFunc = myOtherFunc;
+myObj.myOtherFunc(); // = "HELLO WORLD!"
+
+// We can also specify a context for a function to execute in when we invoke it
+// using `call` or `apply`.
+
+var anotherFunc = function(s){
+    return this.myString + s;
+}
+anotherFunc.call(myObj, " And Hello Moon!"); // = "Hello World! And Hello Moon!"
+
+// The `apply` function is nearly identical, but takes an array for an argument
+// list.
+
+anotherFunc.apply(myObj, [" And Hello Sun!"]); // = "Hello World! And Hello Sun!"
+
+// This is useful when working with a function that accepts a sequence of
+// arguments and you want to pass an array.
+
+Math.min(42, 6, 27); // = 6
+Math.min([42, 6, 27]); // = NaN (uh-oh!)
+Math.min.apply(Math, [42, 6, 27]); // = 6
+
+// But, `call` and `apply` are only temporary. When we want it to stick, we can
+// use `bind`.
+
+var boundFunc = anotherFunc.bind(myObj);
+boundFunc(" And Hello Saturn!"); // = "Hello World! And Hello Saturn!"
+
+// `bind` can also be used to partially apply (curry) a function.
+
+var product = function(a, b){ return a * b; }
+var doubler = product.bind(this, 2);
+doubler(8); // = 16
+
+// When you call a function with the `new` keyword, a new object is created, and
+// made available to the function via the this keyword. Functions designed to be
+// called like that are called constructors.
+
+var MyConstructor = function(){
+    this.myNumber = 5;
+}
+myNewObj = new MyConstructor(); // = {myNumber: 5}
+myNewObj.myNumber; // = 5
+
+// Every JavaScript object has a 'prototype'. When you go to access a property
+// on an object that doesn't exist on the actual object, the interpreter will
+// look at its prototype.
+
+// Some JS implementations let you access an object's prototype on the magic
+// property `__proto__`. While this is useful for explaining prototypes it's not
+// part of the standard; we'll get to standard ways of using prototypes later.
+var myObj = {
+    myString: "Hello world!"
+};
+var myPrototype = {
+    meaningOfLife: 42,
+    myFunc: function(){
+        return this.myString.toLowerCase()
+    }
+};
+
+myObj.__proto__ = myPrototype;
+myObj.meaningOfLife; // = 42
+
+// This works for functions, too.
+myObj.myFunc(); // = "hello world!"
+
+// Of course, if your property isn't on your prototype, the prototype's
+// prototype is searched, and so on.
+myPrototype.__proto__ = {
+    myBoolean: true
+};
+myObj.myBoolean; // = true
+
+// There's no copying involved here; each object stores a reference to its
+// prototype. This means we can alter the prototype and our changes will be
+// reflected everywhere.
+myPrototype.meaningOfLife = 43;
+myObj.meaningOfLife; // = 43
+
+// We mentioned that `__proto__` was non-standard, and there's no standard way to
+// change the prototype of an existing object. However, there are two ways to
+// create a new object with a given prototype.
+
+// The first is Object.create, which is a recent addition to JS, and therefore
+// not available in all implementations yet.
+var myObj = Object.create(myPrototype);
+myObj.meaningOfLife; // = 43
+
+// The second way, which works anywhere, has to do with constructors.
+// Constructors have a property called prototype. This is *not* the prototype of
+// the constructor function itself; instead, it's the prototype that new objects
+// are given when they're created with that constructor and the new keyword.
+MyConstructor.prototype = {
+    myNumber: 5,
+    getMyNumber: function(){
+        return this.myNumber;
+    }
+};
+var myNewObj2 = new MyConstructor();
+myNewObj2.getMyNumber(); // = 5
+myNewObj2.myNumber = 6
+myNewObj2.getMyNumber(); // = 6
+
+// Built-in types like strings and numbers also have constructors that create
+// equivalent wrapper objects.
+var myNumber = 12;
+var myNumberObj = new Number(12);
+myNumber == myNumberObj; // = true
+
+// Except, they aren't exactly equivalent.
+typeof myNumber; // = 'number'
+typeof myNumberObj; // = 'object'
+myNumber === myNumberObj; // = false
+if (0){
+    // This code won't execute, because 0 is falsy.
+}
+if (Number(0)){
+    // This code *will* execute, because Number(0) is truthy.
+}
+
+// However, the wrapper objects and the regular builtins share a prototype, so
+// you can actually add functionality to a string, for instance.
+String.prototype.firstCharacter = function(){
+    return this.charAt(0);
+}
+"abc".firstCharacter(); // = "a"
+
+// This fact is often used in "polyfilling", which is implementing newer
+// features of JavaScript in an older subset of JavaScript, so that they can be
+// used in older environments such as outdated browsers.
+
+// For instance, we mentioned that Object.create isn't yet available in all
+// implementations, but we can still use it with this polyfill:
+if (Object.create === undefined){ // don't overwrite it if it exists
+    Object.create = function(proto){
+        // make a temporary constructor with the right prototype
+        var Constructor = function(){};
+        Constructor.prototype = proto;
+        // then use it to create a new, appropriately-prototyped object
+        return new Constructor();
+    }
+}
+```
+
+## Further Reading
+
+The [Mozilla Developer
+Network](https://developer.mozilla.org/en-US/docs/Web/JavaScript) provides
+excellent documentation for JavaScript as it's used in browsers. Plus, it's a
+wiki, so as you learn more you can help others out by sharing your own
+knowledge.
+
+MDN's [A re-introduction to
+JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript)
+covers much of the concepts covered here in more detail. This guide has quite
+deliberately only covered the JavaScript language itself; if you want to learn
+more about how to use JavaScript in web pages, start by learning about the
+[Document Object
+Model](https://developer.mozilla.org/en-US/docs/Using_the_W3C_DOM_Level_1_Core)
+
+[Learn Javascript by Example and with Challenges](http://www.learneroo.com/modules/64/nodes/350) is a variant of this reference with built-in challenges. 
+
+[JavaScript Garden](http://bonsaiden.github.io/JavaScript-Garden/) is an in-depth
+guide of all the counter-intuitive parts of the language.
+
+[JavaScript: The Definitive Guide](http://www.amazon.com/gp/product/0596805527/) is a classic guide / reference book. 
+
+In addition to direct contributors to this article, some content is adapted
+from Louie Dinh's Python tutorial on this site, and the [JS
+Tutorial](https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript)
+on the Mozilla Developer Network.