Javascript

Front 1

What is the difference between an event handler returning false and calling Event.preventDefault()?

Back 1

Returning false not only prevents the browser's default behavior but also stops propagation of the event.

Front 2

How do you test that a symbol is undefined?

Back 2

if (typeof mySymbol == "undefined") { /* ... */ }

Note the use of "undefined" as a string.

Front 3

What's the difference between referencing an undefined variable vs. an undefined property of a variable?

Back 3

The former throws a ReferenceError, while the latter resolves to undefined (the only value of the special undefined type).

Front 4

In the jQuery UI $.widget() framework, how do you refer to the jQuery object on which a jQuery UI widget was instantiated?

Back 4

this.element

Note that this yields a *jQuery* object, not a DOM element.

Front 5

What's the signature for setTimeout()?

Back 5

setTimeout(code, milliseconds, [lang])

'code' may be a function or an expression.

http://www.w3schools.com/jsref/met_win_settimeout.asp

Front 6

What's the signature for setInterval()?

Back 6

setInterval(code, milliseconds, [lang])

'code' may be a function or an expression.

http://www.w3schools.com/jsref/met_win_setinterval.asp