Undefined is a variable in global scope. A variable that has not been assigned any value is referred as undefined.

You can check for undefined in various ways.Here are some ways to check :-

1. Checking via Strict equality(===)

var a;
    if(a=== undefined)
{
   // these statements execute
}

In the above code, the variable a is not defined, and the if statement evaluates to true.

2. Checking via typeof operator

var a;
    if(typeof a === 'undefined')
{
   // these statements execute
}

Typeof operator also works for undeclared variables,i.e,it does not throw error even if the variable is not declared.

// a has not been declared before
if (typeof a === 'undefined') { // evaluates to true without errors
   // these statements execute
}

if(a === undefined){ // throws a ReferenceError

}

3. Checking via Void operator

var a;
if (a === void 0) {
   // these statements execute
}

In Javascript void works as an operator,it takes an operand,it has nothing to do with the results and returns undefined.
We should avoid using void because it always evaluates to undefined.