JavaScript Control Structures (Loops and Branches)
JavaScript supports the following forms of if..else statement −
- if statement
- if…else statement
- if…else if… statement.
if statement
The if statement is the fundamental control statement that allows JavaScript to make decisions and execute statements conditionally.
Syntax
The syntax for a basic if statement is as follows −
if (expression){ Statement(s) to be executed if expression is true }
if…else statement:
The ‘if…else’ statement is the next form of control statement that allows JavaScript to execute statements in a more controlled way.
Syntax
if (expression){ Statement(s) to be executed if expression is true } else{ Statement(s) to be executed if expression is false }
Here JavaScript expression is evaluated. If the resulting value is true, the given statement(s) in the ‘if’ block, are executed. If the expression is false, then the given statement(s) in the else block are executed. if...else if... statement The if...else if... statement is an advanced form of if…else that allows JavaScript to make a correct decision out of several conditions. Syntax The syntax of an if-else-if statement is as follows − if (expression 1){ Statement(s) to be executed if expression 1 is true } else if (expression 2){ Statement(s) to be executed if expression 2 is true } else if (expression 3){ Statement(s) to be executed if expression 3 is true } else{ Statement(s) to be executed if no expression is true } There is nothing special about this code. It is just a series of if statements, where each ifis a part of the else clause of the previous statement. Statement(s) are executed based on the true condition, if none of the conditions is true, then the else block is executed.
JavaScript – Switch Case
You can use multiple if...else…if statements, as in the previous chapter, to perform a multiway branch. However,
this is not always the best solution, especially when all of the branches depend on the value of a single variable.
Starting with JavaScript 1.2, you can use a switch statement which handles exactly this situation, and it does so
more efficiently than repeated if...else if statements.
Syntax
The objective of a switch statement is to give an expression to evaluate and several different statements to execute
based on the value of the expression. The interpreter checks each case against the value of the expression until a match is found.
If nothing matches,
a default condition will be used.
switch (expression)
{
case condition 1: statement(s)
break;
case condition 2: statement(s)
break;
...
case condition n: statement(s)
break;
default: statement(s)
}
The break statements indicate the end of a particular case. If they were omitted, the interpreter would continue executing
each statement in each of the following cases.
