Using conditionals is a way to apply logic to your applications such that certain code will be executed only under the correct conditions. You do this by applying comparison logic to variable values. The following sections describe the comparisons available in JavaScript and how to apply them in conditional statements.
A comparison operator evaluates two pieces of data and returns true
if the evaluation is correct and false
if the evaluation is not correct. A comparison operator compares the value on the left of the operator against the value on the right.
Table 2.3 shows a list of the comparison operators, along with some examples.
You can chain together multiple comparisons by using logical operators and standard parentheses. Table 2.4 shows a list of the logical operators and how to use them to chain together comparisons.
An if
statement allows you to separate code execution based on the evaluation of a comparison. The following lines of code show the conditional operators in ()
and the code to execute if the conditional evaluates to true
in {}
:
if(x==5){
do_something();
}
In addition to only executing code within the if
statement block, you can specify an else
block that will be executed only if the condition is false
. For example:
if(x==5){
do_something();
} else {
do_something_else();
}
You can also chain together if
statements. To do this, add a conditional statement along with an else
statement, as in this example:
if(x<5){
do_something();
} else if(x<10) {
do_something_else();
} else {
do_nothing();
}
Another type of conditional logic is the switch
statement. The switch
statement allows you to evaluate an expression once and then, based on the value, execute one of many different sections of code.
The syntax for the switch statement is:
switch(expression){
case value1:
<code to execute>
break;
case value2:
<code to execute>
break;
default:
<code to execute if not value1 or value2>
}
Here is what happens: The switch
statement evaluates the expression entirely and gets a value. The value may be a string, a number, a Boolean, or even an object. The switch
expression is then compared to each value specified by the case
statement. If the value matches, the code in the case
statement is executed. If no values match, then the default code is executed.
Note
Typically each case
statement includes a break
command at the end to signal a break out of the switch
statement. If no break
is found, then code execution continues with the next case
statement.