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98
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1 Objects
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2 =======
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3
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4 JavaScript has objects and is object-oriented, but in an unusual way
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5 (more on that later).
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100
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6
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7
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8 Simple objects
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9 --------------
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10
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11 Simple objects are like dictionaries or hashes in other languages.
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12 They support key-value storage and access to attributes. Here's how
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13 to create a simple object.
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14
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15 .. code-block:: javascript
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16
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17 js> obj = {}
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18 [object Object]
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19
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20 js> typeof obj
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21 object
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22
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23 We can store attribute values in a simple object (or any object in the
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24 doc:`object-tree`).
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25
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26 .. code-block:: javascript
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27
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28 js> obj.s = 'hi'
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29 hi
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30 js> obj.i = 10
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31 10
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32
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33 We can get these values back again.
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34
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35 .. code-block:: javascript
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36
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37 js> obj.s === 'hi'
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38 true
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39 js> obj.i == 10
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40 true
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41
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42
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43 Missing attributes
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44 ------------------
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45
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46 It's not an error to ask for something that's not there. We get
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47 undefined.
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48
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49 .. code-block:: javascript
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50
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51 js> obj.dne === undefined
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52 true
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53
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54 We still get undefined if we set the value to undefined.
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55
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56 .. code-block:: javascript
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57
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58 js> obj.undef = undefined
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59 undefined
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60 js> obj.dne === undefined
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61 true
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62
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63 You can use null to signal that there is a value, but that it is None.
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64
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65 .. code-block:: javascript
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66
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67 js> obj.none = null
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68 null
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69 js> obj.none === null
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70 true
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71
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72
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73 hasOwnProperty
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74 --------------
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75
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76 You can use hasOwnProperty to help figure out why an object has an
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77 undefined attribute. It's also useful when inspecting the
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78 doc::`object-tree`. Often, however, it's better to write your code so
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79 you don't need to do this (for example by using null).
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80
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81 .. code-block:: javascript
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82
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83 js> obj.hasOwnProperty('dne')
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84 false
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85 js> obj.hasOwnProperty('undef')
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86 true
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87 js> obj.hasOwnProperty('none')
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88 true
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89
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90
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91 Object literals
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92 ---------------
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93
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94 You can create a simple object by placing key-value pairs in the curly
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95 braces.
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96
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97 .. code-block:: javascript
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98
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99 js> someone = {
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100 > 'name': 'Joe Doe',
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101 > 'age': 43
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102 > }
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103 [object Object]
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104
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105 js> someone.age
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106 43
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107
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108 Take care *not* to put a trailing semicolon in the object literal. It
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109 will work in Firefox but not in Internet Explorer.
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110
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111
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112 Arrays
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113 ------
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114
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115 An array is a list of items. You can put anything is as a value for
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116 the list. Use square brackets to create an array.
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117
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118 .. code-block:: javascript
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119
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120 js> array = []
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121
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122 Arrays expand to accomodate the data you store in them. You can even
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123 leave gaps.
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124
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125 .. code-block:: javascript
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126
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127 js> array[0] = 'zero'
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128 zero
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129 js> array[3] = 'three'
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130 three
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131
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132 An array turned into a string consists of the string on its entries
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133 joined by commas.
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134
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135 .. code-block:: javascript
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136
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137 js> array
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138 zero,,,three
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139 js> array[1] === undefined
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140 true
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141
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142
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143 Array literals
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144 --------------
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145
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146 As with simple objects, simply place the values between the square
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147 brackets, separated by commas.
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148
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149 .. code-block:: javascript
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150
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151 js> seasons = ['spring', 'summer', 'autumn', 'winter']
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152 spring,summer,autumn,winter
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153
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154
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155 As with simple objects, beware of trailing commas and missing entries.
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156 This will work in some browsers and not others.
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157
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158
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159 JSON
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160 ----
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161
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162 `JSON <http://www.json.org/>`_,stands for JavaScript Simple Object
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163 Notation. It is very much one of the best parts of JavaScript. It is
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164 `the fat-free alternative to XML <http://www.json.org/xml.html>`_, and
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165 is widely used in AJAX (instead of XML). JSON objects are object and
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166 array literals constructed using only simple objects, arrays, strings,
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167 true, false and null.
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168
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169 If your code accepts JSON objects then it will be a lot easier to use
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170 it with AJAX or otherwise integrate it with other systems.
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171
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172 Many programming languages have JSON libraries. You don't have to use
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173 JavaScript to use JSON.
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174
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175 To provide standards, there are rules on how to write a JSON objects.
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176 Many applications and programming languages will generate valid JSON
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177 for you. I find YAML a convenient way of authoring JSON data.
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