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Javascript: Scope
/*In JavaScript, functions are the only things that create a new scope*/
var globalVar = "This is a global variable.";
var globalFunction = function() {
var localVar = "This is a local variable.";
var localFunction = function() {
var localVar = "hello, world!";
alert(localVar);
};
localFunction();
alert(localVar);
};
globalFunction();
var g = "global";
function go() {
var l = "local";
}
go();
alert(l); // throws a reference error
//JavaScript uses the this keyword to get a reference to the current execution context.
function go() { console.debug(this); }
go(); //gives you a reference to the top-level execution context; in a browser, that’s the browser window itself.
var myObject = {
go: function() {
console.debug(this);
}
};
myObject.go(); // console.debugs a reference to myObject
//when using functions as constructors, you see the same behavior:
function MyClass() {
this.go = function() {
console.debug(this);
}
}
var instance1 = new MyClass();
var instance2 = new MyClass();
instance1.go(); // console.debugs a reference to the MyClass instance1
instance2.go(); // console.debugs a reference to the MyClass instance2
//JavaScript functions have two methods available to them that are of particular interest for handling context. Let’s look at call:
showCount.call(products, 4);
//Apply is very similar but is used when you don’t know how many arguments you will be passing. It takes an array as its second parameter:
showCount.apply(products, [4]);
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