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7.3 Data Type Conversion

Microsoft JScript provides automatic type conversion as the context may require. This means that if the context expects a value to be a string, for example, JScript tries to convert the value to a string.
 
The language has six types of data. All values have one of these types:
 
undefined
The undefined type has one value only, undefined.
Null
The null type has one value only, null.
Boolean
The Boolean type represents the two logical values, true and false.
String
A string delineated by single or double quotation marks; can contain zero or more unicode characters. An empty string ("") has zero characters and length.
Number
Can be an integer or floating point number according to the IEEE 754 specification. There also several special values:
  • NaN, or not a Number
  • Positive Infinity
  • Negative Infinity
  • Positive zero
  • Negative zero
Object
An object definition including its set of properties and methods.
The following table defines what happens when the context requires that JScript convert one data type into another:
 

Output Input
  Undefined Null Boolean Number String Object
Boolean false
 
false
 
no conversion
 
false if +0, -0 or NaN, otherwise true
 
false if empty string (""), otherwise true
 
true
 
Number NaN
 
NaN
 
1 if true +0 if false
 
no conversion
 
If it cannot be interpreted as a number, it is interpreted as NaN
 
Number object
 
String undefined
 
"null"
 
"true" or "false"
 
The absolute value of the number plus its sign, with the following exceptions:
NaN returns "NaN" +0 or -0 returns "0" + infinity returns "Infinity" - infinity returns "-Infinity"

 
no conversion
 
String object
 
Object runtime error
 
runtime error
 
New Boolean object
 
New Number object
 
New String object
 
no conversion
 


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