Based on the EcmaScript specification, there are certain object properties that are supposed to be undeletable due to the internal parameter DontDelete. For example:
var y = 5
should not be able to be deleted. However, upon further investigation, it seems that it can be deleted.
Refer to the Mozilla Developer Center for more information: https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special/delete
Any thoughts on why this functionality is not behaving as expected?
It's important to verify the information you come across. The ECMA-specification (262, ed 5) does not include an internal parameter called DontDelete. Perhaps they are referring to the [Configurable] property instead? Remember, the delete operator is used for object properties, not variables or functions:
var y=5,
z = {y:5};
delete y;
delete z.y;
alert(y); //=> 5
alert(z.y); //=> undefined
My response to this topic led to the discussion in this Stack Overflow question, where T.J. Crowder provided an insightful answer.
As per the specifications in ES5 table 17:
CreateMutableBinding(N, D) This method creates a new mutable binding within an environment record. The value of N represents the name of the binding. If the optional Boolean argument D is set to true, then the binding may be deleted at a later time.
Furthermore, in section 10.5 Declaration Binding Instantiation
- For each VariableDeclaration and VariableDeclarationNoIn found in the code, proceed in source text order as follows: [...] ii. Invoke the env’s SetMutableBinding method with the parameters dn, undefined, and strict.
This information implies that declared variables are intended to be non-deletable. In the case of global code, the activation object is the global object, which acts as the variable object. Therefore, declared global variables should not be deletable. However, it is worth noting that some browsers may not strictly adhere to this rule.
let x = 10;
alert(delete(x));
Display true. Afterwards, cannot be deleted.
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