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JavaScript Data Types

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What Are the JavaScript Data Types?
Microsoft JavaScript has six types of data. The main types are numbers, strings, objects, and Booleans. The other two are null and undefined.
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String Data Type
Strings are delineated by single or double quotation marks. (Use single quotes to type strings that contain quotation marks.) A string is also an object in JavaScript, but it is a special case, with special properties. The following are examples of strings:
"The cow jumped over the moon." 
'"Avast, ye lubbers!" roared the technician.' 
"42"
A string can contain zero or more unicode characters. When it contains zero, it is called a zero-length string ("").
Number Data Type
JavaScript supports both integer and floating-point numbers. Integers can be positive, 0, or negative; a floating-point number can contain either a decimal point, an "e" (uppercase or lowercase), which is used to represent "ten to the power of" in scientific notation, or both. These numbers follow the IEEE 754 standard for numerical representation. Last, there are certain number values that are special: Integers can be represented in base 10 (decimal), base 8 (octal), and base 16 (hexadecimal).

Octal integers are specified by a leading "0", and can contain digits 0 through 7. If a number has a leading "0" but contains the digits "8" and/or "9", it is a decimal number. A number that would otherwise be an octal number but contains the letter "e" (or "E") generates an error.

Hexadecimal ("hex") integers are specified by a leading "0x" (the "X" can be uppercase or lowercase) and can contain digits 0 through 9 and letters A through F (either uppercase or lowercase). The letter "e" is a permissible digit in hexadecimal notation and does not signify an exponential number. The letters A through F are used to represent, as single digits, the numbers that are 10 through 15 in base 10. That is, 0xF is equivalent to 15, and 0x10 is equivalent to 16.

Octal and hexadecimal numbers can be negative, but cannot be fractional. A number that begins with a single "0" and contains a decimal point is a decimal floating-point number; if a number that begins with "0x" or "00" contains a decimal point, anything to the right of the decimal point is ignored.

Some example numbers:

.0001, 0.0001, 1e-4, 1.0e-4  // Four floating-point numbers, equivalent to each other.
3.45e2                       // A floating-point number, equivalent to 345.
42                           // An integer number.
0377                         // An octal integer, equivalent to 255.
00.0001                      // As octal numbers cannot have decimal parts, this is equivalent to 0.
0378                         // An integer, equivalent to 378.
0Xff                         // A hexadecimal integer, equivalent to 255.
0x37CF                       // A hexadecimal integer, equivalent to 14287.
0x3e7                        // A hexadecimal integer, equivalent to 999.
0x3.45e2                     // As hexadecimal numbers cannot have decimal parts, this is equivalent to 3.
Booleans
The possible Boolean values are true and false. These are special values, and are not usable as 1 and 0.


Note  In a comparison, any expression that evaluates to 0 is taken to be false, and any statement that evaluates to a number other than 0 is taken to be true. Thus the following expression evaluates to true:


(false == 0)

For more information on comparisons, see Controlling Program Flow.

Undefined Data Type
A value that is undefined is simply a value given to a variable after it has been created, but before a value has been assigned to it.
Null Data Type
A null value is one that has no value and means nothing.

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