CONTROL STRUCTURES:
The control structures
within JavaScript allow the program flow to change within a unit of code or
function. These statements can determine whether or not given statements are
executed, as well as repeated execution of a block of code.
There are three ways
to change the control structures of a program by using:
1.conditional
statements-( IF..THEN, . SWITCH)
2. looping control-(.FOR, WHILE, DO..WHILE)
3.branch logic-(FUNCTIONS
AND PROCEDURES)
IF …THEN:
Sytanx: if (test condition
) {
Statements to be executed if the
test condition is true
}
Description:The if statement
is straight forward - if the given expression is true, the statement or
statements will be executed. Otherwise, they are skipped.
Example:
if (a === b) {
document.body.innerHTML += "a equals b";
}
IF..THEN…ELSE:
Syntax:
If (test condition_1){
Statements to be executed if test
condition_1 is true
}else if(test condition_2){
Statements to be executed
if test condition_2 is true
}else{
Statements to be executed
if test condition_2 is false
}
Description: The if statement
may also consist of multiple parts, incorporating else and else
if sections.
These keywords are part of the if statement, and identify the code
blocks that are executed if the preceding condition is false.
Example:
if (a === b) {
document.body.innerHTML += "a equals b";
} else if (a === c) {
document.body.innerHTML += "a equals c";
} else {
document.body.innerHTML += "a does not equal either b or c";
}
SWITCH:
Syntax:
switch(i) {
case 1:
// ...
break;
case 2:
// ...
break;
default:
// ...
break;
}
Description:
- The switch statement evaluates an expression, and determines flow control based on the result of the expression:
- When i gets evaluated, it's value is checked against each of the case labels. These case labels appear in the switch statement and, if the value for the case matches i, continues the execution at that point. If none of the case labels match, execution continues at the default label (or skips the switch statement entirely if none is present.)
- Case labels may only have constants as part of their condition.
- The break keyword exits the switch statement, and appears at the end of each case in order to prevent undesired code from executing.
- The continue keyword does not apply to switch statements.
Example:
var trees
= "green";
switch (trees) {
case "purple":
alert("Trees are purple");
break;
case "pink":
alert("Trees are pink");
break;
case "green":
alert("Trees are green");
break;
default:
alert("Trees are an unknown colour");
}
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