JavaScript Reference: Statements and Control Flow

if

• Use: Execute a block of code if a given condition (or set of conditions) is true
• Format: if ( condition ) { block to execute if condition is true }
• Example:

  var x = 15
  var y = 3
  
  if ( x > y ) { 
  	console.log( "x is bigger" )
  }
  

if...else

• Use: Execute a block of code if a condition is true. If not true, execute a different block
• Format: if ( condition ) { block to execute if condition is true } else { block to execute if condition is false }
• Example:

  var x = 15
  var y = 3
  
  if ( x < y ) { 
  	console.log( "x is smaller" )
  }
  else {
  	console.log( "x must be at least as big as y")
  }
  

if...else Shorthand Notation

• Use: An alternate construction for if...else statements
• Format: ( condition ) ? (block to execute if condition is true) : (block to execute if condition is false)
• Example:

  var x = 15
  var y = 3
  
  ( x < y ) ? console.log( "x is smaller" ) : console.log( "x must be at least as big as y")
  

if...else if

• Use: Execute a block of code if a condition is true. If not true, test a second condition and execute a different block. Any number of conditions can be tested for, and else can be used to run a block if all conditions are false.
• Format: if ( condition ) { statement } else if ( second condition ) { second statement }
• Example:

  var x = 15
  var y = 3
  
  if ( x < y ) { 
  	console.log ( "x is smaller" )
  }
  else if ( x > y ) {
  	console.log ( "x is bigger")
  }
  else {
  	console.log ( "x must be equal to y")
  }
  

for

• Use: Execute a block of code a fixed number of times.
• Format: for ( initial value; condition; value increment ) { statement }
• Example:

  for ( var i=0; i < 5; i++ ) {
  	console.log( "This message will print until i is 5, but right now it's only " + i)
  }
  

  would result in the following output:
  This message will print until i is 5, but right now it's only 0
  This message will print until i is 5, but right now it's only 1
  This message will print until i is 5, but right now it's only 2
  This message will print until i is 5, but right now it's only 3
  This message will print until i is 5, but right now it's only 4

for...in

• Use: Execute a block of code for each element in an array, or the names of properties of an object.
• Format: for ( element in array ) { statement }
• Example: A for loop over array elements.

  var breeds = ["Lhasa Apso","Shih Tzu","Shiba Inu"]
  for ( b in breeds ) {
  	console.log( b )
  }

  would result in the following output:
  Lhasa Apso
  Shih Tzu
  Shiba Inu
• Example: A for loop over object elements.

  var dog1 = { breed:"Dalmation", name:"Spot", color:"Black and white" }
  for ( x in dog1 ) {
  	console.log( dog1[x] + ' is my ' + x )
  }

  would result in the following output:
  Dalmation is my breed
  Spot is my name
  Black and white is my color

while

• Use: Execute a statement as long as a condition is true. Note that the condition is tested before the block is executed.
• Format: while ( condition ) { statement }
• Example:

  var x = 0
  
  while (x <= 5) {
     console.log( "The remainder of " + x + " when divided by 3 is " + x%3) 
     x += 1
  }

  would result in the following output:
  The remainder of 0 when divided by 3 is 0
  The remainder of 1 when divided by 3 is 1
  The remainder of 2 when divided by 3 is 2
  The remainder of 3 when divided by 3 is 0
  The remainder of 4 when divided by 3 is 1
  The remainder of 5 when divided by 3 is 2
• Example:

  var x = 1
  
  while (x < 1) {
     console.log( x )
     x += 1
  }

  would result in no output.

do...while

• Use: Execute a statement as long as a condition is true. Note that the condition is tested after the block is executed.
• Format: do { statement } while ( condition )
• Example:

  var x = 0
  
  do {
     console.log( "The remainder of " + x + " when divided by 3 is " + x%3) 
     x += 1
  }
  while (x <= 5)

  would result in the following output:
  The remainder of 0 when divided by 3 is 0
  The remainder of 1 when divided by 3 is 1
  The remainder of 2 when divided by 3 is 2
  The remainder of 3 when divided by 3 is 0
  The remainder of 4 when divided by 3 is 1
  The remainder of 5 when divided by 3 is 2
• Example:

  var x = 1
  
  do {
     console.log( x )
     x += 1
  }
  while (x < 1)

  would result in the following output:
  1

• Use: Execute a block of code based on the value (case) of an expression. Frequently paired with break so the switch stops execution once a match is found. An optional default statement will run if no match is found.
• Format: switch ( expression ) { case case_1: statement_1; case case_2: statement_2 }
• Example:

  var breed = prompt("What kind of dog do you have?")
  
  switch ( breed ) {
  	case "English Bulldog":
  		alert("Bulldogs are courageous but gentle and were bred to bait bulls.")
  		break;
  	case "Clumber Spaniel":
  		alert("Clumber spaniels are calm but affectionate and originated in France.")
  		break;
  	case "Labrador Retriever":
  		alert("Labs are athletic and are excellent swimmers.")
  		break;
  	case "Papillon":
  		alert("Papillons are small but intelligent dogs with long hair and big ears.")
  		break;
  	default:
  		alert("Sorry, I don't know about your dog!")
  }
  

  Try it

break

• Use: Exit a loop if a condition is true
• Example:

  var t = [1,2,3,4,5,6,7,8,9,10]
  
  // the loop below searches through the array, but ends the search when 5 is found
  
  for (i in t) {
      if (i == 5) { 
      	console.log("Found it"); 
      	break; 
      }
      else { 
      	console.log("Haven't found 5 yet")
      }
  }
  

continue

• Use: Skip an iteration of a loop (and continue to next) if a condition is true
• Example:

  var t = [1,2,3,4,5,6,7,8,9,10]
  
  // the loop below prints all entries of the array except 4:
  
  for (i in t) {
      if (i == 4) { 
      	continue
      }
      else { 
      	console.log( i )
      }
  }