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| author | Radomir Dopieralski <radomir.dopieralski@allegro.pl> |
|---|---|
| date | Wed, 27 Jun 2012 16:35:54 +0200 |
| parents | 29c7fe19aeb5 |
| children |
| rev | line source |
|---|---|
| 104 | 1 The global object |
| 2 ================= | |
| 3 | |
| 127 | 4 JavaScript has a global object. It is, so to speak, the object of |
| 5 last resort. If there's no other suitable object then JavaScript will | |
| 6 use the global object (rather than reporting an error). | |
| 7 | |
| 8 .. note:: | |
| 9 | |
|
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10 Douglas Crockford writes **JavaScript's global object [...] is far and |
| 127 | 11 away the worst part of JavaScript's many bad parts.** |
| 104 | 12 |
| 13 | |
| 14 Getting global | |
| 15 --------------- | |
| 16 | |
| 127 | 17 Here's how to get the global object. |
| 18 | |
| 104 | 19 .. code-block:: javascript |
| 20 | |
| 21 js> return_this = function(){return this;} | |
| 22 | |
| 23 function () { | |
| 24 return this; | |
| 25 } | |
| 26 | |
| 27 js> global = return_this() | |
| 28 [object global] | |
| 127 | 29 |
| 30 The programmer who wants to can always obtain access to the global | |
| 31 object. | |
| 32 | |
| 33 | |
| 34 Global variables | |
| 35 ---------------- | |
| 36 | |
| 37 Global variables are nothing more than attributes of the global | |
| 38 object. | |
| 39 | |
| 40 .. code-block:: javascript | |
| 41 | |
| 104 | 42 js> s = 'Hello world.' |
| 43 Hello world. | |
| 44 js> global.s | |
| 45 Hello world. | |
| 46 js> global.i = 42 | |
| 47 42 | |
| 48 js> i | |
| 49 42 | |
| 127 | 50 |
| 51 | |
| 52 Global pollution | |
| 53 ---------------- | |
| 54 | |
| 55 It's very easy to inadvertently pollute (change) the global object. | |
| 56 All it takes is an assignment to an undeclared variable in a function. | |
| 57 | |
| 58 .. code-block:: javascript | |
| 59 | |
| 104 | 60 js> pollute = function(n){ i = n; }; |
| 61 function (n) { | |
| 62 i = n; | |
| 63 } | |
| 64 js> i | |
| 65 42 | |
| 66 js> pollute(13) | |
| 67 js> i | |
| 68 13 | |
| 127 | 69 |
| 70 More global pollution | |
| 71 --------------------- | |
| 72 | |
| 73 There's a more subtle way to pollute the global object, which involves :doc:`JavaScript's this object <what-is-this>`. | |
| 74 | |
| 75 To begin with, note that we can push values onto an array. | |
| 76 | |
| 77 .. code-block:: javascript | |
| 78 | |
| 104 | 79 js> array = [] |
| 80 | |
| 81 js> array.push(1, 2, 3) | |
| 82 3 | |
| 83 js> array | |
| 84 1,2,3 | |
| 127 | 85 |
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Radomir Dopieralski <radomir.dopieralski@allegro.pl>
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86 Let's try using ``array.push`` as a stand-alone function. |
| 127 | 87 |
| 88 .. code-block:: javascript | |
| 89 | |
| 104 | 90 js> p = array.push |
| 91 function push() { [native code for Array.push, arity=1] } | |
| 92 | |
| 93 js> p(4, 5, 6) | |
| 94 3 | |
| 127 | 95 |
| 96 The push function returns the length of the object it has just pushed | |
| 97 to. So the value *3* above is a signal that all is not well. And | |
| 98 indeed it is not. The original array is unchanged, and the global | |
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Mark inline code using duble backticks
Radomir Dopieralski <radomir.dopieralski@allegro.pl>
parents:
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|
99 object has an entry at *0*. |
| 127 | 100 |
| 101 .. code-block:: javascript | |
| 102 | |
| 104 | 103 js> array |
| 104 1,2,3 | |
| 105 js> | |
| 106 js> global[0] | |
| 107 4 | |
| 127 | 108 |
| 109 It's also got a length! | |
| 110 | |
| 111 .. code-block:: javascript | |
| 112 | |
| 113 js> global.length | |
| 114 3 | |
| 115 | |
| 116 | |
| 117 Explanation | |
| 118 ----------- | |
| 119 | |
| 120 This behaviour is a consequence of :doc:`what-is-this` and | |
| 121 :doc:`bind-is-transient`, together with JavaScript's no-exceptions | |
| 122 design. |