Collected JavaScript Interview Questions

What are global variables? How are they declared? What are the problems with using globals?

From http://www.techrepublic.com/blog/software-engineer/javascript-interview-questions-and-answers/

Global variables are available throughout your code: that is, the variables have no scope. Local variables scope, on the other hand, is restricted to where it is declared (like within a function). The var keyword is used to declare a local variable or object, while omitting the var keyword creates a global variable.

Most JavaScript developers avoid globals. One reason why is they're averse to naming conflicts between local and globals, Also, code that depends on globals can be difficult to maintain and test.

When would you use var in your declaration and when you wouldn’t?

From http://doppnet.com/2011/10/10-advanced-javascript-interview-questions/

Always use var. Not using var for variable declaration will traverse scopes all the way up till the global scope. If variable with that name is not found it will declare it in the global scope. Therefore not using var implicitly declares variable in the global scope

What is the difference between == and ===? Which one would you use?

The equality (==) operator will compare for equality after doing necessary type casting, the identity operator (===) doesn't do any conversions. A good practice suggested by Douglas Crockford is to always use strict equality.

How would you check if a variable is null/undefined?

//check if bar is null bar === null

//check if bar is undefined typeof bar === "undefined"

How do you check if a variable is an object
You can use typeof to determine if variable is an object, however bear in mind that null is actually an object! However null object is 'falsy' thus the following will work:

if(bar && typeof bar === "object") { console.log('bar is object and is not null'); }