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JavaScript Tutorial >> Conditions in JavaScript.


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Why we need JavaScript Conditions?

Fairly often, we need a JavaScript to do different things under different conditions. For example, we might write a JavaScript that checks the time every hour, and changes some parameter appropriately during the course of the day. We might write a JavaScript that can accept some sort of input, and act accordingly. Or we might write a JavaScript that repeats a specified action.

There are several kinds of JavaScript conditions that we can test. All conditional tests in Microsoft JavaScript are Boolean, so the result of any test is either true or false. We can freely test values that are of Boolean, numeric, or string type here in this section of JavaScript Tutorial.

JavaScript provides control structures for a range of possibilities. The simplest control structures are the conditional statements in JavaScript.

Explanation of JavaScript Conditions

JavaScript supports if and if...else conditional statements. In if statements a condition is tested, and if the condition meets the test, some JavaScript code we've written is executed. In the if...else statement, different code is executed if the condition fails the test. The simplest form of an if statement can be written entirely on one line, but multilane if and if... else statements are much more common in web development.

The following JavaScript Conditions example demonstrate syntaxes we can use with if and if...else statements. The first example shows the simplest kind of Boolean test. If (and only if) the item between the parentheses evaluates to true, the statement or block of statements after the if is executed.

Example of JavaScript Conditions

// The ifTest() function is defined elsewhere in the code.
if (male)
   forMaleOnly(" Male Test of If is true.");  // Boolean test of whether (male) is true.

// In this example, the test fails unless both conditions are true.
if (selected.color == "deep green " && selected.texture == "medium crumple")
{
        theAnswer = ("Is it a nice flower? <br> ");
}

// In this example, the test succeeds if either condition is true.
var theAnswer = "";
if ((lbsWeight > 15) || (lbsWeight > 45))
{
    theAnswer = ("Oh, what a cute kitty! <br>");
}
else
    theAnswer = ("That's one huge cat you've got there! <br>");

JavaScript Conditional Operator

JavaScript also supports an implicit conditional form. It uses a question mark (?) after the condition to be tested rather than the word if before the condition, and specifies two alternatives, one to be used if the condition is met and one if it is not. The alternatives are separated by a colon here in JavaScript Conditional Operator.

Example of JavaScript Conditional Operator

var hours = "";

// Code specifying that hours contains either the contents of
// theHour, or theHour - 12.

hours += (theHour >= 12) ? " PM" : " AM";
document.write (hours);

Tip If we have several JavaScript conditions to be tested together, and we know that one is more likely to pass or fail than any of the others, depending on whether the tests are connected with JavaScript OR (||) or JavaScript AND (&&), we can speed execution of our JavaScript by putting that condition first in the conditional statement. For example, if three JavaScript conditions must all be true using && JavaScript operators and the second test fails, the third JavaScript condition is not tested.

Similarly, if only one of several conditions must be true using JavaScript || operators, testing stops as soon as any one condition passes the test. This is particularly effective if the conditions to be tested involve execution of JavaScript function calls or other code.

An example of the side effect of short-circuiting is that runsecond will not be executed in the following example if runfirst() returns 0 or false as shown below of our JavaScript Tutorial.

if ((runfirst() == 0) || (runsecond() == 0))

// some JavaScript code

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JavaScript If Else and Switch Statements Examples



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