Javascript if/switch/with

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The if statement has the following syntax:

if (condition) 
   statement1 
else 
   statement2

The condition can be any expression. Javascript converts the expression into a Boolean using Boolean() function.

If the condition evaluates to true, statement1 is executed; if the condition evaluates to false, statement2 is executed.

Example 1

In the following code the conditional expression compares the value myAge against a numeric value of 13:

var myAge = 12;
if(myAge <  13) { //w  w  w .  ja va 2  s .co  m
   console.log("under 13.");
} 

var i = 3;
if (i){
   console.log("true"); 
}else {
   console.log("false");
} 

The code above generates the following result.

if else ladder

You can also chain if statements together.

if (condition1) 
   statement1 
else if (condition2) 
   statement2 
else 
   statement3

Example 2

Here's an example:

var i = 50;
if (i > 25) {
   console.log("Greater than 25."); 
} else if (i < 0) {
   console.log("Less than 0."); 
} else {
   console.log("Between 0 and 25, inclusive."); 
} 

The code above generates the following result.

switch

The syntax for the switch statement in JavaScript:

switch (expression) { 
    case value: statement 
        break; 
    case value: statement 
        break; 
    case value: statement 
        break; 
    case value: statement 
        break; 
    default: statement 
}

The following code:

if (i == 2){ 
    console.log("2"); 
} else if (i == 3) { 
    console.log("3"); 
} else if (i == 4) { 
    console.log("4"); 
} else { 
    console.log("Other"); 
}

can be rewritten as follows:

switch (i) { 
    case 2: 
        console.log("2"); 
        break;
    case 3: 
        console.log("3"); 
        break;
    case 4: 
        console.log("4"); 
        break;
    default: 
        console.log("Other"); 
}

Putting a break statement after each case avoids having cases fall through into the next one.

A fall-through case statement:

switch (i) { 
    case 2: 
    case 3: /* falls through */
        console.log("2 or 3"); 
        break;
    case 4: 
        console.log("4"); 
        break;
    default: 
        console.log("Other"); 
}

The switch statement works with all data types and the case values need not be constants.

switch ("hello world") {
    case "hello" + " world": 
        console.log("hello"); 
        break;
    case "goodbye": 
        console.log("see you"); 
        break;
    default: 
        console.log("Unexpected message was found."); 
}

Example

Case expressions with comparison operators:

var num = 2; 
switch (true) { 
    case num < 0: 
        console.log("Less than 0."); 
        break; 
    case num >= 0 && num <= 10: 
        console.log("Between 0 and 10."); 
        break; 
    case num > 10 && num <= 20: 
        console.log("Between 10 and 20."); 
        break; 
    default: 
        console.log("More than 20."); 
}

The switch statement compares values using the identically equal operator, therefore "10" is not equal to 10.

with

The with statement sets the scope within a particular object. The syntax is as follows:

with (expression) 
   statement;

For the following code,

var qs = location.search.substring(1); 
var hostName = location.hostname;

The code above can be rewritten using the with statement:

with(location){ 
    var qs = search.substring(1); 
    var hostName = hostname; 
}

In strict mode, the with statement is not allowed and is considered a syntax error.

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