1. Difference between
window.onload and onDocumentReady?
The onload event does not fire until every last piece of the
page is loaded, this includes css and images, which means there’s a huge delay
before any code is executed.
That isnt what we want. We just want to wait until the DOM
is loaded and is able to be manipulated. onDocumentReady allows the programmer
to do that.
2. What is the
difference between == and === ?
The == checks for value equality, but === checks for both
type and value.
3. What does “1″+2+4
evaluate to? What about 5 + 4 + “3″?
Since 1 is a string, everything is a string, so the result
is 124. In the second case, its 93.
4. What is the
difference between undefined value and null value?
undefined means a variable has been declared but has not yet
been assigned a value. On the other hand, null is an assignment value. It can
be assigned to a variable as a representation of no value.
Also, undefined and null are two distinct types: undefined
is a type itself (undefined) while null is an object.
Unassigned variables are initialized by JavaScript with a
default value of undefined. JavaScript never sets a value to null. That must be
done programmatically.
5. How do you change
the style/class on any element?
document.getElementById(“myText”).style.fontSize = “20″;
-or-
document.getElementById(“myText”).className = “anyclass”;
6. What are
Javascript closures?When would you use them?
Two one sentence summaries:
* a closure is the local variables for a function – kept
alive after the function has returned, or
* a closure is a stack-frame which is not deallocated when
the function returns.
A closure takes place when a function creates an environment
that binds local variables to it in such a way that they are kept alive after
the function has returned. A closure is a special kind of object that combines
two things: a function, and any local variables that were in-scope at the time
that the closure was created.
The following code returns a reference to a function:
function sayHello2(name) {
var text = ‘Hello ‘ + name; // local variable
var sayAlert = function() { alert(text); }
return sayAlert;
}
Closures reduce the need to pass state around the
application. The inner function has access to the variables in the outer
function so there is no need to store the information somewhere that the inner
function can get it.
This is important when the inner function will be called
after the outer function has exited. The most common example of this is when
the inner function is being used to handle an event. In this case you get no
control over the arguments that are passed to the function so using a closure
to keep track of state can be very convenient.
7. What is
unobtrusive javascript? How to add behavior to an element using javascript?
Unobtrusive Javascript refers to the argument that the
purpose of markup is to describe a document’s structure, not its programmatic
behavior and that combining the two negatively impacts a site’s
maintainability. Inline event handlers are harder to use and maintain, when one
needs to set several events on a single element or when one is using event
delegation.
1
<input type="text" name="date" />
Say an input field with the name “date” had to be validated
at runtime:
1
document.getElementsByName("date")[0].
2
addEventListener("change", validateDate, false);
3
4
function validateDate(){
5
// Do something when the content of the 'input' element with
the name 'date' is changed.
6
}
Although there are some browser inconsistencies with the
above code, so programmers usually go with a javascript library such as JQuery
or YUI to attach behavior to an element like above.
8. What is Javascript namespacing? How and where
is it used?
Using global variables in Javascript is evil and a bad
practice. That being said, namespacing is used to bundle up all your
functionality using a unique name. In JavaScript, a namespace is really just an
object that you’ve attached all further methods, properties and objects. It
promotes modularity and code reuse in the application.
9. What datatypes are supported in Javascript?
Number, String, Undefined, null, Boolean
10. What is the
difference between innerHTML and append() in JavaScript?
InnerHTML is not standard, and its a String. The DOM is not,
and although innerHTML is faster and less verbose, its better to use the DOM
methods like appendChild(), firstChild.nodeValue, etc to alter innerHTML
content.
