Data Types (JavaScript)
In JavaScript, there are three primary data types, two composite data types, and two special data types.
A string value is a chain of zero or more Unicode characters (letters, digits, and punctuation marks). You use the string data type to represent text in JavaScript. You include string literals in your scripts by enclosing them in single or double quotation marks. Double quotation marks can be contained in strings surrounded by single quotation marks, and single quotation marks can be contained in strings surrounded by double quotation marks. The following are examples of strings:
"Happy am I; from care I'm free!" '"Avast, ye lubbers!" roared the technician.' "45" 'c'
Notice that JavaScript does not have a type to represent a single character. To represent a single character in JavaScript, you create a string that consists of only one character. A string that contains zero characters ("") is an empty (zerolength) string.
JavaScript provides escape sequences that you can include in strings to create characters that you cannot type directly. For example, \t specifies a tab character. For more information, see Special Characters (JavaScript).
In JavaScript, there is no distinction between integer and floatingpoint values; a JavaScript number can be either (internally, JavaScript represents all numbers as floatingpoint values).
Integer values can be positive whole numbers, negative whole numbers, and 0. They can be represented in base 10 (decimal), base 16 (hexadecimal), and base 8 (octal). Most numbers in JavaScript are written in decimal.
You denote hexadecimal ("hex") integers by prefixing them with a leading "0x" (zero and xX). They can contain digits 0 through 9, and letters A through F (either uppercase or lowercase) only. The letters A through F are used to represent, as single digits, 10 through 15 in base 10. That is, 0xF is equivalent to 15, and 0x10 is equivalent to 16.
You denote octal integers by prefixing them with a leading "0" (zero). They can contain digits 0 through 7 only. A number that has a leading "0" and contains the digits "8" and/or "9" is interpreted as a decimal number.
Both hexadecimal and octal numbers can be negative, but they cannot have a decimal portion, and they cannot be written in scientific (exponential) notation.
Note 

Starting in Internet Explorer 9 standards mode, Internet Explorer 10 standards mode, Internet Explorer 11 standards mode, and Windows Store apps, the parseInt function does not treat a string that has a prefix of "0" as octal. When you are not using the parseInt function, however, strings with a prefix of "0" can still be interpreted as octal. 
Floatingpoint values can be whole numbers with a decimal portion. Additionally, they can be expressed in scientific notation. That is, an uppercase or lowercase "e" is used to represent "ten to the power of". JavaScript represents numbers using the eight byte IEEE 754 floatingpoint standard for numerical representation. This means you can write numbers as large as 1.79769x10308, and as small as 5x10324. A number that contains a decimal point and that has a single "0" before the decimal point is interpreted as a decimal floatingpoint number.
Notice that a number that begins with "0x" or "00" and contains a decimal point will generate an error. Here are some examples of JavaScript numbers.
Number 
Description 
Decimal Equivalent 

.0001, 0.0001, 1e4, 1.0e4 
Four equivalent floatingpoint numbers. 
0.0001 
3.45e2 
A floatingpoint number. 
345 
45 
An integer. 
45 
0378 
An integer. Although this looks like an octal number (it begins with a zero), 8 is not a valid octal digit, so the number is treated as a decimal. 
378 
0377 
An octal integer. Notice that although it only appears to be one less than the number above, its actual value is quite different. 
255 
0.0001 
A floating point number. Even though this begins with a zero, it is not an octal number because it has a decimal point. 
0.0001 
00.0001 
This is an error. The two leading zeros mark the number as an octal, but octals are not allowed a decimal component. 
N/A (compiler error) 
0Xff 
A hexadecimal integer. 
255 
0x37CF 
A hexadecimal integer. 
14287 
0x3e7 
A hexadecimal integer. Notice that the 'e' is not treated as exponentiation. 
999 
0x3.45e2 
This is an error. Hexadecimal numbers cannot have decimal parts. 
N/A (compiler error) 
Additionally, JavaScript contains numbers with special values. These are:

NaN (not a number). This is used when a mathematical operation is performed on inappropriate data, such as strings or the undefined value

Positive Infinity. This is used when a positive number is too large to represent in JavaScript

Negative Infinity. This is used when a negative number is too large to represent in JavaScript

Positive and Negative 0. JavaScript differentiates between positive and negative zero.
Whereas the string and number data types can have a virtually unlimited number of different values, the Boolean data type can only have two. They are the literals true and false. A Boolean value is a truthvalue: it specifies whether the condition is true or not.
Comparisons you make in your scripts always have a Boolean outcome. Consider the following line of JavaScript code.
y = (x == 2000);
Here, the value of the variable x is compared to the number 2000. If it is, the result of the comparison is the Boolean value true, which is assigned to the variable y. If x is not equal to 2000, then the result of the comparison is the Boolean value false.
Boolean values are especially useful in control structures. The following code combines a comparison that creates a Boolean value directly with a statement that uses it. Consider the following JavaScript code sample.
if (x == 2000) { z = z + 1; } else { x = x + 1; }
The if/else statement in JavaScript performs one action if a Boolean value is true (z = z + 1), and an alternate action if the Boolean value is false (x = x + 1).
You can use any expression as a comparative expression. Any expression that evaluates to 0, null, undefined, or an empty string is interpreted as false. An expression that evaluates to any other value is interpreted as true. For example, you could use an expression such as:
// This may not do what you expect. See below! if (x = y + z)
Note that the above line does not check whether x is equal to y + z, since only a single equal sign (the assignment operator) is used. Instead, the code above assigns the value of y + z to the variable x, and then checks whether the result of the entire expression (the value of x) is zero. To check whether x is equal to y + z, you need to use the following code.
// This is different from the code above! if (x == y + z)
For more information on comparisons, see Controlling Program Flow.
The null data type has only one value in JavaScript: null. The null keyword cannot be used as the name of a function or variable.
A variable that contains null contains no valid Number, String, Boolean, Array, or Object. You can erase the contents of a variable (without deleting the variable) by assigning it the null value.
Notice that in JavaScript, null is not the same as 0 (as it is in C and C++). Also note that the typeof operator in JavaScript reports null values as being of type Object, not of type null. This potentially confusing behavior is for backwards compatibility.
The undefined value is returned when you use an object property that does not exist, or a variable that has been declared, but has never had a value assigned to it.
you can check to see if a variable exists by comparing it to undefined, although you can check if its type is undefined by comparing the type of the variable to the string "undefined". The following example shows how to find out the variable x has been declared:
var x; // This method works. if (x == undefined) { document.write("comparing x to undefined <br/>"); } . // This method doesn't work  you must check for the string "undefined". if (typeof(x) == undefined) { document.write("comparing the type of x to undefined <br/>"); } // This method does work. if (typeof(x) == "undefined") { document.write("comparing the type of x to the string 'undefined'"); } // Output: // comparing x to undefined // comparing the type of x to the string 'undefined'
You can also compare the undefined value to null. This comparison is true if the property someObject.prop is null or if the property someObject.prop does not exist.
someObject.prop == null;
To find out whether an object property exists, you can use the in operator:
if ("prop" in someObject) // someObject has the property 'prop'