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Przetłumaczone getting-started
author Radomir Dopieralski <sheep-devel@sheep.art.pl>
date Sun, 15 Jul 2012 13:27:27 +0200
parents ff23e32bbe29
children
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1 Equality
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2 ========
103
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3
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4 Equality in JavaScript can be a little odd.
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5
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6
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7 Double equals
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8 -------------
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9
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10 JavScript has two operations for equality (and inequality). One of
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11 them is '==' (and '!=' for inequality). It's the oddest.
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12
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13
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14 Not transitive
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15 ^^^^^^^^^^^^^^
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16
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17 If *a* is equal to *b*, and if *b* is equal to *c* , then we expect
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18 *a* to be equal to *c*. This is called the transitive property.
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19
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20
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21 .. code-block:: javascript
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22
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23
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24 js> '0' == 0 // 'a' is equal to 'b'
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25 true
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26 js> 0 == '' // 'b' is equal to 'c'
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27 true
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28 js> '0' == '' // 'a' is not equal to 'c'
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29 false
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30
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31
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32
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33 Not reflexive
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34 ^^^^^^^^^^^^^
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35
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36 We expect *a* to be equal to *a*. This is called the reflexive
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37 property.
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38
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39 .. code-block:: javascript
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40
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41 js> NaN == NaN
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42 false
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43
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44 Fortunately, this seems to be the only example.
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45
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46
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47 Is symmetric
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48 ^^^^^^^^^^^^
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49
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50 If *a* is equal to *b* then we expect *b* to be equal to *a*. This is
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51 called the symmetric property. In JavaScript equality is symmetric.
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52
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53
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54
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55 Triple equality
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56 ---------------
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57
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58 JavaScript also has '===' (and '!==' for inequality).
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59
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60
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61 Is transitive
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62 ^^^^^^^^^^^^^
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63
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64 Unlike double equals, triple equals is always transitive.
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65
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66 .. code-block:: javascript
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67
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68 js> '0' === 0 // unequal
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69 false
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70 js> 0 === '' // unequal
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71 false
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72 js> '0' === '' // unequal
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73 false
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74
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75
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76 Not reflexive
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77 ^^^^^^^^^^^^^
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78
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79 .. code-block:: javascript
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80
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81 js> NaN === NaN
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82 false
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83
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84
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85 By the way, there's a thread on Facebook with subject `Time and Date
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86 on my wall shows NaNNaNNaN at NaN:NaN
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87 <http://www.facebook.com/topic.php?uid=4915599711&topic=54061>`_. I
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88 wonder how that happened.
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89
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90 Is symmetric
130
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91 ^^^^^^^^^^^^
103
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92
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93 Triple equality is still symmetric.
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94
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95
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96 Don't use double equality
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97 -------------------------
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98
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99 Double equality does implicit conversions and besides has some odd
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100 rules. My advice is **don't use double equality**.
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101
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102 Triple equality does not do conversions. If you want to do
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103 conversions in your comparision my advice is to make them explicit.
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104
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105
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106 Compare as string
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107 -----------------
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108
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109 The easiest conversion is to string. Here are two immutables, a
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110 number and a string.
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111
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112 .. code-block:: javascript
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113
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114 js> a = 0
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115 0
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116 js> b = '0'
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117 0
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118
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119
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120 These quantities are double equal but not triple equal.
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121
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122 .. code-block:: javascript
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123
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124 js> a == b
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125 true
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126 js> a === b
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127 false
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128
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129 Here's how to do an explicit conversion to string before comparison,
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130 which gives an equality.
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131
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132 .. code-block:: javascript
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133
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134 js> '' + a === '' + b // Converts variables to strings.
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135 true
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136
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137 .. note::
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138
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139 It's always easier to read a triple equal comparison, because
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140 you're not distracted by the complex double equal rules.
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141
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142
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143 Compare as number
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144 -----------------
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145
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146 [To follow later.]