JavaScript Development Technology Encyclopedia-3rd Chapter JS Data type _ basic knowledge

3.1 Basic data types
1. String type: Consists of Unicode characters, numbers, punctuation marks, no char character type in JavaScript, even if only one character, must be used
2 digit (number): does not distinguish between integral and floating-point types
3. Booleans (Boolean)
3.2 Composite data types
1. Array: Array elements: The data placed in an array is called an array element.
Array subscript: Numbering starting from 0
READ: Arr[0];
Assign value: arr[1]=120;
2. Objects:
The name of the object. Property name
Object name. Method Name ()
3.3 Other data types
Function:
A function in 1.javascript is a data type, so it can be stored in a variable, array, or object.
2 null: In JavaScript, NULL is a standalone data type that represents no value
3.undefined is undefined meaning, there are 4 kinds of cases will return to undefined.
A. Define a variable, but not yet assigned a value.
B. Use of an undefined variable
C. Defines an object property that does not exist
D. A reference to an array element that does not exist.
3.4 Conversion of data types
Principle: Converts a data type to the type of data that is required in the current environment.
JavaScript data type implicit conversion rules
1. Implicit conversions of data types:

Original data type		The data type of the target ticket	Description
String	Non-empty string	Digital Lane	If the character in the string is a number, the number converted to a string, such as "123" converts to a number of 123, If the character in the string is not a pure number, it is converted to NAA
		Boolean type	True
	Empty character	Digital type	0
		Boolean type	Flase
	String "true"	Digital type	Nan
		Boolean type	True
	Field character string "Flase"	Digital type	Nan
		Boolean type	True
Digital type	General numbers	String	Can convert a number to a string "number"
		Boolean type	Convert number 0 to false, other numbers to true
	Nan	String type	Convert to String "Nan"
		Boolean type	Convert to False
Boolean type	True	String type	Convert to String "true"
		Digital type	Convert to 1
	False	String type	Convert to String "false"
		Digital rationale	Convert to 0
Null		String type	Convert to string "null"
		Digital type	Convert to 0
		Boolean type	Convert to False
Undefined		String type	Convert to string "undefined"
		Digital type	Convert to Nan
		Boolean type	Convert to False

2. Implicit conversion of alternative data

3. Display conversions for data types

1. Convert to Digital type

。 Nubmer (value) converts value to an integer or floating-point number

。 parseint (value) converts value to an integer

。 Parseflaot (value) to float

cannot be converted to a numeric type, will return Nan

Conversion rules that convert other types to numeric

td> description

data type
string	empty string	use number () to convert to 0, using parseint () and parsefloat () Convert to Nan
	string "ture"	to nan
	string "False	" to Nan
	Other strings	If the character in the string is a number, convert the number in the character, such as the string "123" to the number 123. If the string is not a pure number, it is converted to Nan
	true	uses the numbers () conversion to 0, using the Parsei NT () and parsefloat () convert to Nan
	false	use number () to convert to 0, using parseint () and parsefloat () to NAN
null		converts to 0 using number () and parseint () and parsefloat () to Nan
undefined		convert to nan

2. Convert to Boolean

To convert another data type to a Boolean

;                      data type		language
string	empty string	convert to false
	string "True"	convert to True
	string "flase"	convert to TRUE
	other string	convert to True
digital	0	convert to False
	Nan	converts to False
	Other numbers	convert to True
null		convert to False
undefined		convert to False

3. Convert to String type

To convert another data type to a string type

                                                                  

td> description

data type
digital	Normal number	converts a number to a string "number", such as a number 1 to a string "1"
	nan	to String "nan"
Boolean	true	convert string "true"
	flase	to String "false"
null		to string "null"
undefined		to string "undefined"