吴裕雄--天生自然PHP-MySQL-JavaScript学习笔记：JavaScript中的表达式和控制流

<script>
  month = "July"
  if (month == "October") document.write("It's the fall")
</script>

<script>
  a = 1000
  b = "1000"
  if (a == b)  document.write("1")
  if (a === b) document.write("2")
</script>

<script>
  a = 7; b = 11
  if (a > b)  document.write("a is greater than b<br>")
  if (a < b)  document.write("a is less than b<br>")
  if (a >= b) document.write("a is greater than or equal to b<br>")
  if (a <= b) document.write("a is less than or equal to b<br>")
</script>

<script>
  a = 1; b = 0
  document.write((a && b) + "<br>")
  document.write((a || b) + "<br>")
  document.write((  !b  ) + "<br>")
</script>

<script>
  if (finished == 1 || getnext() == 1) done = 1
</script>

<script>
  gn = getnext()
  if (finished == 1 OR gn == 1) done = 1;
</script>

<script>
  string = "The quick brown fox jumps over the lazy dog"

  with (string)
  {
    document.write("The string is " + length + " characters<br>")
    document.write("In upper case it's: " + toUpperCase())
  }
</script>

<script>
  onerror = errorHandler
  document.writ("Welcome to this website") // Deliberate error

  function errorHandler(message, url, line)
  {
    out  = "Sorry, an error was encountered.

";
    out += "Error: " + message + "
";
    out += "URL: "   + url     + "
";
    out += "Line: "  + line    + "

";
    out += "Click OK to continue.

";
    alert(out);
    return true;
  }
</script>

<script>
  try
  {
    request = new XMLHTTPRequest()
  }
  catch(err)
  {
    // Use a different method to create an XML HTTP Request object
  }
</script>

<script>
  if      (page == "Home")  document.write("You selected Home")
  else if (page == "About") document.write("You selected About")
  else if (page == "News")  document.write("You selected News")
  else if (page == "Login") document.write("You selected Login")
  else if (page == "Links") document.write("You selected Links")
</script>

<script>
  switch (page)
  {
    case "Home":
      document.write("You selected Home")
      break
    case "About":
      document.write("You selected About")
      break
    case "News":
      document.write("You selected News")
      break
    case "Login":
      document.write("You selected Login")
      break
    case "Links":
      document.write("You selected Links")
      break
  }
</script>

<script>
  switch (page)
  {
    case "Home":
      document.write("You selected Home")
      break
    case "About":
      document.write("You selected About")
      break
    case "News":
      document.write("You selected News")
      break
    case "Login":
      document.write("You selected Login")
      break
    case "Links":
      document.write("You selected Links")
      break
    default:
      document.write("Unrecognized selection")
      break
  }
</script>

<script>
  document.write(
    a <= 5 ?
    "a is less than or equal to 5" :
    "a is greater than 5"
  )
</script>

<script>
  counter=0

  while (counter < 5)
  {
    document.write("Counter: " + counter + "<br>")
    ++counter
  }
</script>

<script>
  count = 1

  do
  {
    document.write(count + " times 7 is " + count * 7 + "<br>")
  } while (++count <= 7)
</script>

<script>
  for (count = 1 ; count <= 12 ; ++count)
  {
    document.write(count + " times 12 is " + count * 12 + "<br>");
  }
</script>

<script>
  haystack     = new Array()
  haystack[17] = "Needle"

  for (j = 0 ; j < 20 ; ++j)
  {
    if (haystack[j] == "Needle")
    {
      document.write("<br>- Found at location " + j)
      break
    }
    else document.write(j + ", ")
  }
</script>

<script>
  haystack     = new Array()
  haystack[4]  = "Needle"
  haystack[11] = "Needle"
  haystack[17] = "Needle"

  for (j = 0 ; j < 20 ; ++j)
  {
    if (haystack[j] == "Needle")
    {
      document.write("<br>- Found at location " + j + "<br>")
      continue
    }

    document.write(j + ", ")
  }
</script>

<script>
  document.write("a: " + (42 > 3) + "<br>")
  document.write("b: " + (91 < 4) + "<br>")
  document.write("c: " + (8 == 2) + "<br>")
  document.write("d: " + (4 < 17) + "<br>")
</script>

<script>
  myname = "Peter"
  myage  = 24
  document.write("a: " + 42     + "<br>") // Numeric literal
  document.write("b: " + "Hi"   + "<br>") // String literal
  document.write("c: " + true   + "<br>") // Constant literal
  document.write("d: " + myname + "<br>") // String variable
  document.write("e: " + myage  + "<br>") // Numeric variable
</script>