The JavaScript Diaries: Part 5

By Lee Underwood

Conditional Expressions

There is another method for writing conditional statements, so long as it is a true or false statement. This method uses the conditional operator ( ?: ). Consider the following script:

  var yourAge = prompt("Please enter your age:", "");

  if (yourAge >= 18) {
    alert("You are a grownup");
  }
  else {
    alert("You are a minor");
  }

We've seen this type of script before so we don't need to go into how it works. Using a conditional statement, the same script could be written:

  var yourAge = prompt("Please enter your age:", "");
  var yourStatus = (yourAge >= 18) ? "grownup" : "minor"
  alert("Your are a " + yourStatus)

Here, we declare and initialize the variable yourAge, as in the previous script, but in this case, the entire if/else statement is accomplished on one line. The second line declares and assigns a value to the variable yourStatus depending upon the conditional statement that follows it. The conditional statement reads: "If the variable yourAge is greater than or equal to 18 then the value of the variable yourStatus is 'grownup,' otherwise the value is 'minor.'"

This can save a lot of time and effort in coding a script. Remember though, it only works if you have a true/false statement.

Nesting

A conditional statement can also be contained within another conditional statement. This is called nesting. Consider the following script (with credit to John Pollock):

  var have_cookbook="yes";
  var meatloaf_recipe="yes";

  if (have_cookbook=="yes") {
    if (meatloaf_recipe=="yes") {
      alert("Recipe found");
    }
    else {
      alert("Have the book but no recipe");
    }
  }
  else {
    alert("You need a cookbook");
  }

Let's look at how this script is executed:

  1. Two variables, have_cookbook and meatloaf_recipe, are declared and initialized, each with a value of "yes."
  2. The conditional statement begins by asking if the variable have_cookbook is equal to "yes." (Remember from our previous section, the equal sign [ = ] is an assignment operator and the double equal sign [ == ] is a comparison operator.)
  3. If the statement is true, then another nested conditional statement begins. It asks if the variable meatloaf_recipe is equal to "yes."
    1. If that statement is true, an alert window will display, "Recipe found." Another way of saying it is: "If the variable have_cookbook is equal to 'yes' and the variable meatloaf_recipe is also equal to 'yes,' then display an alert window that says 'Recipe found.'"
    2. If the statement is false, an alert window will display, "Have the book but no recipe." Another way of saying it is: "If the variable have_cookbook' is equal to 'yes' and the variable meatloaf_recipe is equal to no, then display an alert window that says 'Have the book but no recipe.'"
  4. If the variable have_cookbook is not equal to "yes," then the nested statement will be skipped altogether and the final else statement will be executed, displaying an alert window that says, "You need a cookbook."

It sounds a little complicated but if you look closely you'll see it's pretty straightforward.

You could also nest a conditional statement within the bottom (outside) else block. In other words, the same nested block that you see above could also be done in the other part of the statement:

  var have_cookbook = prompt('Do you have a cookbook?','');
  var meatloaf_recipe = prompt('Do you have a meatloaf recipe?','');
  var have_web_access = prompt('Do you have access to the Web?','');

  if (have_cookbook=="yes") {
    if (meatloaf_recipe=="yes") {
      alert("Recipe found");
    }
    else {
      alert("Have the book but no recipe");
    }
  }
  else {
    if (have_web_access=="yes") {
      alert("Find the recipe on the Web");
    }
    else {
      alert("You need a good cookbook");
    }
  }

There's no limit to the number of nested statements you can use. While they seem to be a bit confusing, if taken a step at a time, they can be a very powerful tool.




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