**How to convert a string to a number using
JavaScript?**

You can use the parseInt() and parseFloat() methods. Notice that extra
letters following a valid number are ignored, which is kinda wierd but
convenient at times.

parseInt("100") ==> 100

parseFloat("98.6") ==> 98.6

parseFloat("98.6 is a common temperature.") ==> 98.6

parseInt("aa") ==> Nan //Not a Number

parseInt("aa",16) ==> 170 //you can supply a radix or base

**How to convert numbers to strings using JavaScript? **

You can prepend the number with an empty string

var mystring = ""+myinteger;

or

var mystring = myinteger.toString();

You can specify a base for the conversion,

var myinteger = 14;

var mystring = myinteger.toString(16);

mystring will be "e".

**How to test for bad numbers using JavaScript? **

the global method, "isNaN()" can tell if a number has gone bad.

var temperature = parseFloat(myTemperatureWidget.value);

if(!isNaN(temperature)) {

alert("Please enter a valid temperature.");

}

**What's Math Constants and Functions using JavaScript? **

The Math object contains useful constants such as Math.PI, Math.E

Math also has a zillion helpful functions.

Math.abs(value); //absolute value

Math.max(value1, value2); //find the largest

Math.random() //generate a decimal number between 0 and 1

Math.floor(Math.random()*101) //generate a decimal number between 0 and
100

**What's the Date object using JavaScript? **

Time inside a date object is stored as milliseconds since Jan 1, 1970.

new Date(06,01,02) // produces "Fri Feb 02 1906 00:00:00 GMT-0600
(Central Standard Time)"

new Date(06,01,02).toLocaleString() // produces "Friday, February 02,
1906 00:00:00"

new Date(06,01,02) - new Date(06,01,01) // produces "86400000"

**What does the delete operator do? **

The delete operator is used to delete all the variables and objects used
in the program ,but it does not delete variables declared with var
keyword.

**How tp create Arrays using JavaScript ?**

<script type="text/javascript">

var days = new Array();

days[0] = "Sunday"

days[1] = "Monday"

days[2] = "Tuesday"

days[3] = "Wednesday"

days[4] = "Thursday"

days[5] = "Friday"

days[6] = "Saturday"

document.write("first day is "+days[0])

</script>

This produces

first day is Sunday

A more compact way of creating an array is the literal notation:

<script type="text/javascript">

var days = ["Sunday","Monday","Tuesday","Wednesday", "Thursday","Friday","Saturday"];

document.write("first day is "+days[0]) </script>

This produces

first day is Sunday

**How to delete an entry using JavaScript? **

The "delete" operator removes an array element, but oddly does not
change the size of the array.

<script type="text/javascript">

var days = ["Sunday","Monday","Tuesday","Wednesday", "Thursday","Friday","Saturday"];

document.write("Number of days:"+days.length); delete days[4];

document.write("<br />Number of days:"+days.length);

</script>

This produces

Number of days:7

Number of days:7

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