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author Radomir Dopieralski <radomir.dopieralski@allegro.pl>
date Wed, 27 Jun 2012 16:35:54 +0200
parents a808230bc5dd
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).
103
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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 Is symmetric
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86 ^^^^^^^^^^^^
103
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87
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88 Triple equality is still symmetric.
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89
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90
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91 Don't use double equality
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92 -------------------------
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93
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94 Double equality does implicit conversions and besides has some odd
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95 rules. My advice is **don't use double equality**.
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96
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97 Triple equality does not do conversions. If you want to do
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98 conversions in your comparision my advice is to make them explicit.
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99
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100
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101 Compare as string
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102 -----------------
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103
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104 The easiest conversion is to string. Here are two immutables, a
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105 number and a string.
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106
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107 .. code-block:: javascript
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108
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109 js> a = 0
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110 0
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111 js> b = '0'
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112 0
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113
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114
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115 These quantities are double equal but not triple equal.
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116
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117 .. code-block:: javascript
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118
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119 js> a == b
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120 true
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121 js> a === b
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122 false
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123
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124 Here's how to do an explicit conversion to string before comparison,
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125 which gives an equality.
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126
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127 .. code-block:: javascript
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128
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129 js> '' + a === '' + b // Converts variables to strings.
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130 true
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131
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132 .. note::
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133
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134 It's always easier to read a triple equal comparison, because
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135 you're not distracted by the complex double equal rules.
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136
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137
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138 Compare as number
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139 -----------------
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140
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141 There is a way to make sure that a value is a number in JavaScript:
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142 use the unary plus operator:
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143
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144 .. code-block:: javascript
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145
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146 js> +a === +b
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147 true
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148
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149 When the value cannot be converted to a number, you will get a ``NaN`` value.
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150 `NaN` is never equal to anything, even to itself.
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151
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152 Alternatively, you can use the ``parseInt`` and ``parseFloat`` functions to convert
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153 strings to number -- their behavior is slightly different, as they will silently
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154 ignore all non-number characters in the string.
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155
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156 .. code-block:: javascript
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157
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158 js> parseInt('3abc1')
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159 3
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160 js> +'3abc1'
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161 NaN
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162
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163 Because ``NaN`` is never equal to anything, even itself, you cannot check whether
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164 you got ``NaN`` as a result with ``==`` or ``===``. You have to use the ``isNaN``
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165 function for that.
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166
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167 Compare as boolean
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168 ------------------
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169
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170 To convert an expression to a boolean value, it's most convenient to use the
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171 ``!`` boolean negation operator twice, like this:
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172
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Radomir Dopieralski <radomir.dopieralski@allegro.pl>
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173 .. code-block:: javascript
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Radomir Dopieralski <radomir.dopieralski@allegro.pl>
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174
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175 js> !!0 === !!''
25a718801405 Add "Compare as number" and "Compare as boolean"
Radomir Dopieralski <radomir.dopieralski@allegro.pl>
parents: 130
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176 true