Mercurial > js4py
annotate core-javascript/objects.rst @ 213:0ea90a1819c2 default tip
Custom theming
| author | Radomir Dopieralski <radomir.dopieralski@allegro.pl> |
|---|---|
| date | Wed, 27 Jun 2012 16:35:54 +0200 |
| parents | 92c876338c31 |
| children |
| rev | line source |
|---|---|
| 98 | 1 Objects |
| 2 ======= | |
| 3 | |
| 4 JavaScript has objects and is object-oriented, but in an unusual way | |
| 5 (more on that later). | |
| 100 | 6 |
| 7 | |
| 8 Simple objects | |
| 9 -------------- | |
| 10 | |
| 11 Simple objects are like dictionaries or hashes in other languages. | |
| 12 They support key-value storage and access to attributes. Here's how | |
| 13 to create a simple object. | |
| 14 | |
| 15 .. code-block:: javascript | |
| 16 | |
| 17 js> obj = {} | |
| 18 [object Object] | |
| 19 | |
| 20 js> typeof obj | |
| 21 object | |
| 22 | |
| 23 We can store attribute values in a simple object (or any object in the | |
| 24 doc:`object-tree`). | |
| 25 | |
| 26 .. code-block:: javascript | |
| 27 | |
| 28 js> obj.s = 'hi' | |
| 29 hi | |
| 30 js> obj.i = 10 | |
| 31 10 | |
| 32 | |
| 33 We can get these values back again. | |
| 34 | |
| 35 .. code-block:: javascript | |
| 36 | |
| 37 js> obj.s === 'hi' | |
| 38 true | |
| 39 js> obj.i == 10 | |
| 40 true | |
| 41 | |
| 42 | |
| 43 Missing attributes | |
| 44 ------------------ | |
| 45 | |
| 46 It's not an error to ask for something that's not there. We get | |
| 47 undefined. | |
| 48 | |
| 49 .. code-block:: javascript | |
| 50 | |
| 51 js> obj.dne === undefined | |
| 52 true | |
| 53 | |
| 54 We still get undefined if we set the value to undefined. | |
| 55 | |
| 56 .. code-block:: javascript | |
| 57 | |
| 58 js> obj.undef = undefined | |
| 59 undefined | |
| 60 js> obj.dne === undefined | |
| 61 true | |
| 62 | |
| 63 You can use null to signal that there is a value, but that it is None. | |
| 64 | |
| 65 .. code-block:: javascript | |
| 66 | |
| 67 js> obj.none = null | |
| 68 null | |
| 69 js> obj.none === null | |
| 70 true | |
| 71 | |
| 72 | |
| 73 hasOwnProperty | |
| 74 -------------- | |
| 75 | |
| 76 You can use hasOwnProperty to help figure out why an object has an | |
| 77 undefined attribute. It's also useful when inspecting the | |
| 78 doc::`object-tree`. Often, however, it's better to write your code so | |
| 79 you don't need to do this (for example by using null). | |
| 80 | |
| 81 .. code-block:: javascript | |
| 82 | |
| 83 js> obj.hasOwnProperty('dne') | |
| 84 false | |
| 85 js> obj.hasOwnProperty('undef') | |
| 86 true | |
| 87 js> obj.hasOwnProperty('none') | |
| 88 true | |
| 89 | |
| 90 | |
| 91 Object literals | |
| 92 --------------- | |
| 93 | |
| 94 You can create a simple object by placing key-value pairs in the curly | |
| 95 braces. | |
| 96 | |
| 97 .. code-block:: javascript | |
| 98 | |
| 99 js> someone = { | |
| 100 > 'name': 'Joe Doe', | |
| 101 > 'age': 43 | |
| 102 > } | |
| 103 [object Object] | |
| 104 | |
| 105 js> someone.age | |
| 106 43 | |
| 107 | |
|
201
d4753a63211b
semicolon -> comma in object literals
Radomir Dopieralski <radomir.dopieralski@allegro.pl>
parents:
100
diff
changeset
|
108 Take care *not* to put a trailing comma in the object literal. It |
| 100 | 109 will work in Firefox but not in Internet Explorer. |
| 110 | |
| 111 | |
| 112 Arrays | |
| 113 ------ | |
| 114 | |
|
202
37b3b4948387
Remove spurious "is"
Radomir Dopieralski <radomir.dopieralski@allegro.pl>
parents:
201
diff
changeset
|
115 An array is a list of items. You can put anything as a value for |
| 100 | 116 the list. Use square brackets to create an array. |
| 117 | |
| 118 .. code-block:: javascript | |
| 119 | |
| 120 js> array = [] | |
| 121 | |
| 122 Arrays expand to accomodate the data you store in them. You can even | |
| 123 leave gaps. | |
| 124 | |
| 125 .. code-block:: javascript | |
| 126 | |
| 127 js> array[0] = 'zero' | |
| 128 zero | |
| 129 js> array[3] = 'three' | |
| 130 three | |
| 131 | |
| 132 An array turned into a string consists of the string on its entries | |
| 133 joined by commas. | |
| 134 | |
| 135 .. code-block:: javascript | |
| 136 | |
| 137 js> array | |
| 138 zero,,,three | |
| 139 js> array[1] === undefined | |
| 140 true | |
| 141 | |
| 142 | |
| 143 Array literals | |
| 144 -------------- | |
| 145 | |
| 146 As with simple objects, simply place the values between the square | |
| 147 brackets, separated by commas. | |
| 148 | |
| 149 .. code-block:: javascript | |
| 150 | |
| 151 js> seasons = ['spring', 'summer', 'autumn', 'winter'] | |
| 152 spring,summer,autumn,winter | |
| 153 | |
| 154 | |
| 155 As with simple objects, beware of trailing commas and missing entries. | |
| 156 This will work in some browsers and not others. | |
| 157 | |
| 158 | |
| 159 JSON | |
| 160 ---- | |
| 161 | |
|
203
92c876338c31
Remove the remark about YAML, as YAML is not JSON
Radomir Dopieralski <radomir.dopieralski@allegro.pl>
parents:
202
diff
changeset
|
162 `JSON <http://www.json.org/>`_ stands for JavaScript Simple Object |
| 100 | 163 Notation. It is very much one of the best parts of JavaScript. It is |
| 164 `the fat-free alternative to XML <http://www.json.org/xml.html>`_, and | |
| 165 is widely used in AJAX (instead of XML). JSON objects are object and | |
| 166 array literals constructed using only simple objects, arrays, strings, | |
| 167 true, false and null. | |
| 168 | |
| 169 If your code accepts JSON objects then it will be a lot easier to use | |
| 170 it with AJAX or otherwise integrate it with other systems. | |
| 171 | |
| 172 Many programming languages have JSON libraries. You don't have to use | |
| 173 JavaScript to use JSON. | |
| 174 | |
| 175 To provide standards, there are rules on how to write a JSON objects. | |
| 176 Many applications and programming languages will generate valid JSON | |
|
203
92c876338c31
Remove the remark about YAML, as YAML is not JSON
Radomir Dopieralski <radomir.dopieralski@allegro.pl>
parents:
202
diff
changeset
|
177 for you. |