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Przetłumaczone getting-started
author Radomir Dopieralski <sheep-devel@sheep.art.pl>
date Sun, 15 Jul 2012 13:27:27 +0200
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children
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1 The object tree
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2 ===============
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4 Objects can have attributes. The statements
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6 .. code-block:: javascript
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8 value = obj.attr # Get the attr of 'a'.
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9 obj.attr = value # Set the attr of 'a'.
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11 respectively **set** and **get** the *attr* attribute of the object
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12 *obj*. Inheritance is where an object gets some its attributes from
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13 one or other more objects. JavaScript uses an object tree for
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14 inheritance.
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17 .. _inheritance-tree:
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19 Tree
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20 ----
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21
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22 All JavaScript objects are part of an inheritance tree. Each object
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23 in the tree has a parent object, which is also called the prototype
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24 object (of the child). There is a single exception to this rule,
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25 which is the **root** of the tree. The root of the tree does not have
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26 a parent object.
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28 .. note::
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29
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30 **You can't get far in JavaScript without understanding the
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31 object tree.**
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34 Get
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35 ^^^
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36
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37 When JavaScript needs to get an attribute value of an object, it first
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38 looks up the attr of the attribute in the object's dictionary. If the
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39 attr is a key in the dictionary, the associated value is returned.
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41 If the attr is not a key, then the process is repeated using the
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42 object's parent, grandparent, and so on until the key is found. If
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43 the key is not found in this way then *undefined* is returned.
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46 Set
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47 ^^^
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49 When JavaScript needs to set an attribute value of an object it
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50 ignores the inheritance tree. It simply sets that value in the
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51 object's dictionary.
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54 Root
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55 ----
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56
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57 When the interpreter starts up, the **root** of the tree is placed at
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58 *Object.prototype*. (We'll find out later why that location is used.)
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59
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60 Every object inherits from the **root**, although perhaps not
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61 directly. Here's a simple example:
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63 .. code-block:: javascript
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65 js> root = Object.prototype
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66 js> a = {}
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67 js> a.attr === undefined
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68 true
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69 js> root.attr = 'gotcha'
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70 js> a.attr
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71 gotcha
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72
102
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73 If we give *root* a *attr* attribute then every other object,
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74 including those already created and those not yet created, also has a
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75 *attr* attribute with the same value. (In practice it's better not to
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76 change Object.prototpye.)
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77
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78 This applies arrays:
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80 .. code-block:: javascript
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81
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82 js> array = [0, 1, 2]
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83 0,1,2
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84 js> array.attr
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85 gotcha
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86
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87 And even to functions:
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89 .. code-block:: javascript
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90
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91 js> f = function(){}
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92 function () {
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93 }
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94
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95 js> f.attr
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96 gotcha
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97
102
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98 .. note::
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99
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100 A page might have many scripts, all of which would like to modify
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101 the Object.prototype root object. This can cause bugs and
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102 incompatibilities. So try not to do this.
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103
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104
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105 However, this up-the-tree lookup does not apply if *attr* is found
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106 earlier in the tree. We continue the previous example to show this,
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107 and the behaviour of set.
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108
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109 .. code-block:: javascript
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110
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111 js> a.attr = 'fixed'
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112 js> a.attr
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113 fixed
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114 js> root.attr
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115 gotcha
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116
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117
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118 Create
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119 ------
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120
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121 Any tree can be constructed from its root, together with a command
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122 **create(parent)** that returns a new **child** of the given parent
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123 node.
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124
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125 In all but the most recent version of JavaScript the *create* function
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126 is not built in. However, it's easy to write one, once you know
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127 enough JavaScript. Here's how its done in the work folder's
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128 library.js file.
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129
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130 .. literalinclude:: /core-javascript-work/library.js
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131 :language: javascript
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132 :start-after: start create
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133 :end-before: end create
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134
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135
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136 Using create
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137 ------------
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138
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139 Here's an example of its use:
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140
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141 .. code-block:: javascript
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142
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143 js> a = {}
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144 js> b = create(a)
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145 js> a.attr = 'apple'
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146 apple
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147 js> b.attr
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148 apple
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150
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151 And a continuation of the example:
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152
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153 .. code-block:: javascript
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154
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155 js> c = create(b)
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156 js> c.attr
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157 apple
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158 js> b.attr = 'banana'
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159 banana
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160 js> c.attr
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161 banana
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163
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164 .. note::
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165
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166 JavaScript uses an inheritance tree. By using *create*, we can
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4be9f1bf2fd3 Reviewed and revised existing pages, and added some new stub pages.
Jonathan Fine <jfine@pytex.org>
parents: 8
diff changeset
167 create any inheritance tree. All JavaScript objects are in this
4be9f1bf2fd3 Reviewed and revised existing pages, and added some new stub pages.
Jonathan Fine <jfine@pytex.org>
parents: 8
diff changeset
168 tree.