Javascript If Statement

4 Key excerpts on "Javascript If Statement"

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  eBook - ePub

  Processing

  An Introduction to Programming

  • Jeffrey L. Nyhoff, Larry R. Nyhoff(Authors)
  • 2017(Publication Date)
  • Chapman and Hall/CRC

    (Publisher)

  do not perform the action of adding salt.

  Such a conditional action is sometimes represented as a flowchart :

  Similarly, a computer program may include conditional statements. Such statements are performed selectively , only if a certain condition is true.

  The Basic if Statement

  Most programming languages provide a very powerful feature for conditional action: the if statement . In Processing, the basic if statement is typically written in the form

  if ( condition )

  {  Statement(s) to perform if the condition is true }

  Unlike the other statements we have written so far, the if statement is usually written as more than one line of code, and yet it is still considered to be a single statement.

  Let’s take a closer look at the items of information that we need to supply for the general form of the if statement. First, an if statement needs a test condition .

  if (condition )

  This test condition is usually a comparison that will always turn out to be either true or false . This kind of true-or-false comparison is also known as a Boolean expression.* In Processing, the basic operators for comparison, known as the relational operators , are the following:

  > Greater than

  < Less than

  >= Greater than or equal to

  <= Less than or equal to

  !=  Not equal to

  == Equal to

  Second, we need to specify one or more statements that will be performed if —and only if—the condition is true . We will enclose any such statement(s) in a pair of curly braces, creating what is known as a block of statements.

  if (condition )

  {  Statement(s) to perform if the condition is true }

  Basic if Statement Example: Rolling a Die Game

  Let’s return to the die-rolling Roll program from Chapter 3

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  eBook - ePub

  Beginning JavaScript

  • Jeremy McPeak(Author)
  • 2015(Publication Date)
  • Wrox

    (Publisher)

  true , you execute a particular section of code.

  Look at another example. Recall from Chapter 1 the natural English instructions used to demonstrate how code flows. One of these instructions for making a cup of coffee is:
• Has the kettle boiled? If so, then pour water into cup; otherwise, continue to wait.
  This is an example of making a decision. The condition in this instruction is “Has the kettle boiled?” It has a true or false answer. If the answer is true , you pour the water into the cup. If it isn’t true , you continue to wait.
  In JavaScript, you can change the flow of the code’s execution depending on whether a condition is true or false , using an if statement or a switch statement. You look at these shortly, but first we need to introduce some new operators that are essential for the definition of conditions—comparison operators.

Comparison Operators

In Chapter 2 you saw how mathematical functions, such as addition and division, were represented by symbols, such as plus (+ ) and forward slash (/ ), called operators. You also saw that if you want to give a variable a value, you can assign to it a value or the result of a calculation using the equals sign (= ), termed the assignment operator.

Decision making also has its own operators, which enable you to test conditions. Comparison operators, just like the mathematical operators you saw in the preceding chapter, have a left-hand side (LHS) and a right-hand side (RHS), and the comparison is made between the two. The technical terms for these are the left operand and the right operand. For example, the less-than operator, with the symbol < , is a comparison operator. You could write 23 < 45 , which translates as “Is 23 less than 45?” Here, the answer would be true (see Figure 3.1