Conditional execution is used whenever the flow of the program depends on some variables.
Usage
. check if the user input is valid
. has the calculation reached a critical point
. 

if(condition)
 {
 statements // this is executed if condition evaluates to true
 }
// optional
else
 {
 other statements // this is executed if condition evaluates to false
 }

if (myVariable == 10) // use the proper comparision operator (see below)
{
statements if condition is true; // this is executed if myVariable is 10
}
// optional
else
{
statements if condition is false; // this is executed if variable is not 10
}

Accept a form only if the user has filled-in the required email field

if (emailVariable == '') // important : use the == operator to compare, not the = to assign
{
alert("no email given");
}

for a complete check email example click here