JavaScript_Succinctly

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Clearly, the value of this is based on the context in which the function is being called. Consider that both myObject.sayFoo and sayFoo point to the same function. However, depending upon where (i.e. the context) sayFoo() is called from, the value of this is different. If it helps, here is the same code with the head object (i.e. window) explicitly used. Sample: sample101.html window.foo = 'foo'; window.myObject = { foo: 'I am myObject.foo' }; window.sayFoo = function () { console.log(this.foo); }; window.myObject.sayFoo = window.sayFoo; window.myObject.sayFoo(); window.sayFoo(); Make sure that as you pass around functions, or have multiple references to a function, you realize that the value of this will change depending upon the context in which you call the function. Notes All variables except this and arguments follow lexical scope. The this keyword refers to the head object in nested functions You might be wondering what happens to this when it is used inside of a function that is contained inside of another function. The bad news is in ECMA 3, this loses its way and refers to the head object (the window object in browsers), instead of the object within which the function is defined. In the following code, this inside of func2 and func3 loses its way and refers not to myObject but instead to the head object. Sample: sample102.html 106
var myObject = { func1: function () { console.log(this); // Logs myObject. var func2 = function () { console.log(this) // Logs window, and will do so from this point on. var func3 = function () { console.log(this); // Logs window, as it’s the head object. } (); } (); } } myObject.func1(); The good news is that this will be fixed in ECMAScript 5. For now, you should be aware of this predicament, especially when you start passing functions around as values to other functions. Consider the next sample and what happens when passing an anonymous function to foo.func1. When the anonymous function is called inside of foo.func1 (a function inside of a function), the this value inside of the anonymous function will be a reference to the head object. Sample: sample103.html var foo = { func1: function (bar) { bar(); // Logs window, not foo. console.log(this); // The this keyword here will be a reference to the foo object. } } foo.func1(function () { console.log(this) }); Now you will never forget: the this value will always be a reference to the head object when its host function is encapsulated inside of another function or invoked within the context of another function (again, this is fixed in ECMAScript 5). 107
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By Cody Lindley Foreword by Daniel
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Table of Contents About the Author
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Object() object instance properties
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The Story behind the Succinctly Ser
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About the Author Cody Lindley is a
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Advanced technical books written ab
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“Consider that the cody object cr
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Chapter 1 JavaScript Objects Creati
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Using JavaScript, the getGender met
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* The String() constructor function
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new object that is returned from th
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Notes The Math object is the oddbal
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In this snippet, we created a new s
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Verify that literals are created fr
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The primitive values null, undefine
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The concept to take away here is th
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* This will throw an error and not
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copied, but not the actual value. T
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Sample: sample21.html // Primitive
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Notes Careful! If you mutate the na
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console.log(instanceOfCustomObject.
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myObject.prop = 'test'; myArray.pro
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Chapter 2 Working with Objects and
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Sample: sample31.html // Encapsula
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cody['getGender']() // Just slap th
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een defined. You might think that b
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console.log(myArray.toLocaleString(
Page 56 and 57: Sample: sample44.html var cody = {
Page 58 and 59: Sometimes a web browser manufacture
Page 60 and 61: I like this library because it take
Page 62 and 63: console.log(stringObject); // Logs
Page 64 and 65: Chapter 4 Number() Conceptual overv
Page 66 and 67: NEGATIVE_INFINITY POSITIVE_INFINIT
Page 68 and 69: Parameter passed to Boolean() = Mat
Page 70 and 71: console.log(Boolean(Math)); console
Page 72 and 73: var numberObject = new Number(1.100
Page 74 and 75: Chapter 7 Null Conceptual overview
Page 76 and 77: Chapter 8 Undefined Conceptual over
Page 78 and 79: Chapter 9 The Head/Global Object Co
Page 80 and 81: object. } // Logs bar, foo's value
Page 82 and 83: Make sure you understand that the h
Page 84 and 85: Object() parameters The Object() co
Page 86 and 87: Sample: sample72.html var cody = n
Page 88 and 89: Logs 'foo', being found at Object.p
Page 90 and 91: Sample: sample77.html var addFunct
Page 92 and 93: Sample: sample79.html var yelp = f
Page 94 and 95: parameters when defining the functi
Page 96 and 97: Notes The arguments.length property
Page 98 and 99: Invoking a function (function, meth
Page 100 and 101: Self-invoking anonymous function st
Page 102 and 103: the bar variable. All this is to sh
Page 104 and 105: Chapter 12 The this Keyword Concept
Page 108 and 109: Working around the nested function
Page 110 and 111: What you need to learn here is that
Page 112 and 113: var john = new Person(); // No argu
Page 114 and 115: Notes An unlimited number of functi
Page 116 and 117: } (); How is the value of sayHiTex
Page 118 and 119: var parentFunction = function () {
Page 120 and 121: Chapter 14 Function Prototype Prope
Page 122 and 123: Sample: sample119.html var myFunct
Page 124 and 125: In this example, the foo property i
Page 126 and 127: console.log(FooInstance.constructor
Page 128 and 129: Providing a new object as the proto
Page 130 and 131: var chuck = new Person(0, 0); conso
Page 132 and 133: An Array() is just a special type o
Page 134 and 135: It is more common to see an array d
Page 136 and 137: console.log(myArray.length); // Log
Page 138 and 139: while (myArrayLength--) { // If len
Page 140 and 141: ceil() cos() exp() floor() log() ma
Page 142 and 143: That is: a complex value is equal t
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JavaScript_Succinctly

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