Merge pull request BonsaiDen#66 from scriptin/master

delete operator

Author: BonsaiDen

doc/en/core/delete.md

@@ -0,0 +1,87 @@
+## The `delete` Operator
+
+In short, it's *impossible* to delete global variables, functions and some other
+stuff in JavaScript which have a `DontDelete` attribute set.
+
+### Global code and Function code
+
+When a variable or a function is defined in a global 
+or a [function scope](#function.scopes) it is a property of either 
+Activation object or Global object. Such properties have a set of attributes, 
+one of these is `DontDelete`. Variable and function declarations in global 
+and function code always create properties with `DontDelete`, therefore 
+cannot be deleted.
+
+    // global variable:
+    var a = 1; // DontDelete is set
+    delete a; // false
+    a; // 1
+
+    // normal function:
+    function f() {} // DontDelete is set
+    delete f; // false
+    typeof f; // "function"
+
+    // reassigning doesn't help:
+    f = 1;
+    delete f; // false
+    f; // 1
+
+### Explicit properties
+
+There are things which can be deleted normally: these are explicitly set 
+properties.
+
+    // explicitly set property:
+    var obj = {x: 1};
+    obj.y = 2;
+    delete obj.x; // true
+    delete obj.y; // true
+    obj.x; // undefined
+    obj.y; // undefined
+
+In the example above `obj.x` and `obj.y` can be deleted because they have no 
+`DontDelete` atribute. That's why an example below works too.
+
+    // this works fine, except for IE:
+    var GLOBAL_OBJECT = this;
+    GLOBAL_OBJECT.a = 1;
+    a === GLOBAL_OBJECT.a; // true - just a global var
+    delete GLOBAL_OBJECT.a; // true
+    GLOBAL_OBJECT.a; // undefined
+
+Here we use a trick to delete `a`. [`this`](#function.this) here refers 
+to the Global object and we explicitly declare variable `a` as it's property 
+which allows us to delete it.
+
+IE (at least 6-8) has some bugs, so code above doesn't work.
+
+### Function arguments and built-ins
+
+Functions' normal arguments, [`arguments` object](#function.arguments) 
+and built-in properties also have `DontDelete` set.
+
+    // function arguments and properties:
+    (function (x) {
+    
+      delete arguments; // false
+      typeof arguments; // "object"
+      
+      delete x; // false
+      x; // 1
+      
+      function f(){}
+      delete f.length; // false
+      typeof f.length; // "number"
+      
+    })(1);
+
+### Host objects
+    
+Behaviour of `delete` operator can be unpredictable for hosted objects. Due to 
+specification, host objects are allowed to implement any kind of behavior. 
+
+### In conclusion
+
+`delete` operator often has an unexpected behaviour and can be safely used 
+only for dealing with explicitly set properties on normal objects.

doc/en/index.json

@@ -54,7 +54,8 @@
             "articles": [
                 "eval",
                 "undefined",
-                "semicolon"
+                "semicolon",
+                "delete"
             ]
         },
         {