JS: Caveats in Javascript
Every language has exceptional situations to common rules that are explained as part of its specification. They end up as caveats in programmer’s mind.
Interchanging properties as objects and associative array
Javascript objects are property-maps and property names can be accessed using the membership operator. Likewise, objects can be accessed like associative arrays passing in property name as index.
Note that if accessed using membership operator, property name is constrained by identifier token rules.
HasOwnProperty
A reference to JS object-property will follow the chain of prototype
inheritance, so the only way to figure out whether a property is directly
attached to the object is by using hasOwnProperty method call on the
object. But what if the method is overridden accidentally ?
Stacking Calls with setInterval
setInterval will execute the timeout function every X milliseconds, where X
is the timeout value for the subscribed-handler. What happens if handler
function takes more time to complete than the timeout-value ?
You can execute the above program to understand the behavior of timeout handling in Javascript. But the fact is, it is bad programming to use setInterval a.k.a periodic-timeout if we doubt that the handler function is going to take more time than the next timeout. One possible solution is given in the above snippet itself.
Shadowing undefined
Unfortunately, undefined can also be used as identifier.
Semicolons and leading parenthesis
The call to foo and bar are treated as single statement without a semicolon
in-between, foo()(bar(10)).
Deleting property names
It is not possible to delete a name whose DontDelete attribute is set.
This DontDelete attribute is a feature of javascript. Variables and
function declarations in global and function code always create properties
with DontDelete, and therefore cannot be deleted.
But delete works normal on property types, where names are added
programmatically. This applies even to global this, as demonstrated by the
last block of the following gist.
Late-binding of this
this name is bound late while making function calls.
In the above example, this inside test function is not bound to this
of new Foo() instance when its constructor is called. Instead it is bound
to global object.
Closures and namespace reference
In closures, parent function’s namespaces are only referred by enclosed function, they are not copied to enclosed function’s namespace.
Outputs 10, 10 times.