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author Radomir Dopieralski <radomir.dopieralski@allegro.pl>
date Wed, 27 Jun 2012 16:35:54 +0200
parents 58e4f484903f
children
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1 Solution one
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2 ============
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3
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4 Here's the code we're studying.
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5
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6 .. literalinclude:: /core-javascript-work/exercise-1.js
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7 :language: javascript
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8
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9
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10 Verbal summary
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11 --------------
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12
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13 We are making sure that there's an object AAA and giving it an
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14 attribute AAA.fff, which is a function. We won't say more about the
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15 function in this summary.
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16
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17
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18 Hoisting once-only arguments
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19 ----------------------------
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20
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21 First, we'll hoist the argument to the anonymous function into its
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22 body. We can always do this, without changing the meaning, provided the
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23 function is called only once.
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24
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25 .. literalinclude:: /core-javascript-work/solution-1-a.js
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26 :language: javascript
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27
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28 There, that's better. We no longer have to read to the end of the
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29 function to find out what BBB is (and in particular its relationship
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30 with AAA).
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31
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32
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33 What is AAA?
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34 ------------
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35
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36 Next, we'll hide (or ignore) the assigment to BBB.fff. This
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37 assignment sets an attribute on the object pointed to by the variable
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38 BBB. In other words, it mutates the object pointed to by BBB, but the
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39 variable BBB still points to the same object as it did before.
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40
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41 .. literalinclude:: /core-javascript-work/solution-1-b.js
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42 :language: javascript
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43
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44
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45 If AAA starts out as an object then the expression
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46
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47 .. code-block:: javascript
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48
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49 AAA || {}
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50
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51 evaluates to AAA, while if AAA is undefined then it evaluates to the
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52 newly created object literal (the left-and-right curly braces). In
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53 either case, this value is assigned to the local variable BBB, which
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54 is then returned by the anonymous function and bound to AAA.
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55
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56 Put another way, if AAA starts off as undefined then it is bound to a
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57 new object, while if it starts off as an object then it finishes as
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58 the same object.
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59
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60 Therefore, our truncated version of the original exercise is equivalent to
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61
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62 .. literalinclude:: /core-javascript-work/solution-1-c.js
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63 :language: javascript
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64
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65 This idiom is very common in JavaScript, and is (as here) a
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66 consequence of using objects as namespaces (which is better than using
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67 the global object) and defensiveness about the order in which files
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68 are loaded.
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69
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70 Here's a command line session that shows this equivalence. To begin
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71 with AAA is undefined and the assignment binds AAA to a new object.
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72 For the second assignment AAA is defined and the assignment binds AAA
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73 to the object is already bound to.
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74
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75 .. code-block:: javascript
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76
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77 js> var AAA = AAA || {}
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78 js> AAA
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79 [object Object]
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80 js> pointer = AAA
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81 [object Object]
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82 js> var AAA = AAA || {}
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83 js> pointer === AAA
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84 true
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87 The assignment to BBB.fff
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88 -------------------------
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89
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90 Reading the code, it looks as if BBB is local to the anonymous
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91 function. As a variable it is, but its value is not. The value of
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92 BBB is bound to the global object AAA at the end of the function call.
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93
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94 The following is almost equivalent to our original JavaScript code.
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95 Note that BBB.hhh has been changed to AAA.hhh.
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96
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97 .. literalinclude:: /core-javascript-work/solution-1-d.js
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98 :language: javascript
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99
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100
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101 Caveat
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102 ------
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103
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104 Why did I say *almost equivalent*? Because we might subsequently do
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105 something like
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108 .. code-block:: javascript
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109
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110 var CCC = AAA;
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111 AAA = undefined;
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112
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113 For the original code the closure variable BBB still exists and points
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114 to the same object as CCC, even though AAA is undefined. Thus, the
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115 reference to BBB.hhh in the original code will continue to work.
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116
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117 For the revised code, when the value of AAA is set to undefined the
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118 reference to AAA.hhh will fail.