The if statement executes a statement if a specified condition is true. If the condition is false, another statement can be executed using the else statement..

Note: The else statement is optional.

if (condition)
   statement1
else
   statement2

Multiple if...else statements can be nested to create an else if clause.

if (condition1)
   statement1
else if (condition2)
   statement2
else if (condition3)
   statement3
...
else
   statementN

Note: If you want to execute more than one statement in the if, else or else if part, braces are required around the statements.

MDN link | MSDN link

Using if...else:

if (x == 5) {
  z = 7;
  q = 42;
}
else
  z = 19;

Using else if:

if (x < 10)
  return "Small number";
else if (x < 50)
  return "Medium number";
else if (x < 100)
  return "Large number";
else {
  flag = 1;
  return "Invalid number";
}