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| {{extdev}}
| | See [http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Guide:Variables Javscript 1.5 Guide: Variables] at developer.mozilla.org. |
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| ==Variables in Javascript==
| | [[Category:Redirects]] |
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| ===Scoping===
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| <ol>
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| <li>Declare all variables with '''var''' before assigning to them.
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| Assigning to undeclared variable causes a strict Javascript warning (''"redeclaration of var X"''; you need to set '''javascript.options.strict''' and '''javascript.options.showInConsole''' [[about:config|prefs]] to see it though) and the variable is put in global scope, which is obviously bad.
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| {| border="1" cellpadding="5" rules="all"
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| ! Bad example !! Good example
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| |-
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| <pre>function F() {
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| // generates a strict JS warning
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| x=1;
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| G();
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| // alerts "2", since x is global.
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| alert(x);
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| }
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| function G() {
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| x=2;
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| }</pre>
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| ||<pre>function F() {
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| var x=1;
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| G();
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| alert(x); // alerts "1"
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| }
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| function G() {
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| var x=2;
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| }
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| </pre>
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| |}
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| </li>
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| <li>Declare each variable '''exactly once''' in a function, otherwise you'll get another JS error (''redeclaration of var X'').
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| {| border="1" cellpadding="5" rules="all"
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| ! Good example !! Bad example
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| |-
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| <pre>var i;
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| if(condition) {
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| for(i=0; i<3; i++)
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| dump(i);
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| } else {
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| for(i=2; i>=0; i--)
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| dump(i);
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| }</pre>
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| ||
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| <pre>if(condition) {
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| for(var i=0; i<3; i++)
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| dump(i);
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| } else {
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| // generates a JS warning
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| for(var i=2; i>=0; i--)
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| dump(i);
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| }</pre>
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| |}
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| Due to the fact that '''{}''' blocks don't introduce a new variable scope (all variables declarations are effectively moved to the start of the enclosing function by interpreter), the following two examples are also valid:
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| {| border="1" cellpadding="5" rules="all")
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| ! Kinda ok !! Works, but may become [http://www.mindprod.com/unmain.html unmantainable]
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| |-
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| <pre>if(condition) {
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| for(var i=0; i<3; i++)
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| dump(i);
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| } else {
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| for(i=2; i>=0; i--)
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| dump(i);
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| }
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| </pre>
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| ||
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| <pre>if(condition) {
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| for(i=0; i<3; i++)
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| dump(i);
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| } else {
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| // generates a JS warning
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| for(var i=2; i>=0; i--)
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| dump(i);
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| }</pre>
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| |}
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| </li></ol>
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| [[Category:Development]] [[Category:Javascript|Javascript variables]]
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