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104
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1 The global object
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2 =================
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3
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127
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4 JavaScript has a global object. It is, so to speak, the object of
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5 last resort. If there's no other suitable object then JavaScript will
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6 use the global object (rather than reporting an error).
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7
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8 .. note::
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9
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10 Douglas Crockford writes **JavaScript's global object ... is far and
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11 away the worst part of JavaScript's many bad parts.**
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104
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12
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13
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14 Getting global
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15 ---------------
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16
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127
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17 Here's how to get the global object.
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18
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104
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19 .. code-block:: javascript
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20
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21 js> return_this = function(){return this;}
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22
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23 function () {
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24 return this;
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25 }
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26
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27 js> global = return_this()
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28 [object global]
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127
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29
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30 The programmer who wants to can always obtain access to the global
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31 object.
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32
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33
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34 Global variables
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35 ----------------
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36
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37 Global variables are nothing more than attributes of the global
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38 object.
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39
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40 .. code-block:: javascript
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41
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104
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42 js> s = 'Hello world.'
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43 Hello world.
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44 js> global.s
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45 Hello world.
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46 js> global.i = 42
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47 42
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48 js> i
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49 42
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127
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50
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51
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52 Global pollution
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53 ----------------
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54
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55 It's very easy to inadvertently pollute (change) the global object.
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56 All it takes is an assignment to an undeclared variable in a function.
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57
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58 .. code-block:: javascript
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59
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104
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60 js> pollute = function(n){ i = n; };
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61 function (n) {
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62 i = n;
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63 }
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64 js> i
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65 42
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66 js> pollute(13)
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67 js> i
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68 13
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127
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69
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70 More global pollution
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71 ---------------------
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72
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73 There's a more subtle way to pollute the global object, which involves :doc:`JavaScript's this object <what-is-this>`.
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74
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75 To begin with, note that we can push values onto an array.
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76
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77 .. code-block:: javascript
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78
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104
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79 js> array = []
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80
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81 js> array.push(1, 2, 3)
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82 3
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83 js> array
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84 1,2,3
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127
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85
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86 Let's try using *array.push* as a stand-alone function.
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87
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88 .. code-block:: javascript
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89
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104
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90 js> p = array.push
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91 function push() { [native code for Array.push, arity=1] }
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92
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93 js> p(4, 5, 6)
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94 3
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127
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95
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96 The push function returns the length of the object it has just pushed
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97 to. So the value *3* above is a signal that all is not well. And
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98 indeed it is not. The original array is unchanged, and the global
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99 object has an entry at 0.
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100
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101 .. code-block:: javascript
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102
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104
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103 js> array
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104 1,2,3
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105 js>
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106 js> global[0]
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107 4
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127
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108
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109 It's also got a length!
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110
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111 .. code-block:: javascript
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112
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113 js> global.length
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114 3
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115
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116
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117 Explanation
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118 -----------
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119
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120 This behaviour is a consequence of :doc:`what-is-this` and
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121 :doc:`bind-is-transient`, together with JavaScript's no-exceptions
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122 design.
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