JavaScript Advanced Programming Series-Basic data types

The basic data types in JavaScript include:

Undefined, Null, Boolean, number, String 5 data types

1. Undefined type (only one value Undefined, often used to represent uninitialized variables)

• If a variable is not initialized, its initial value is undefined and the data type is undefined

var MyVar;

MyVar = = = undefined//return True

typeof MyVar = = = ' undefined '//return True

• The parameter list for the function, whose value is undfined for the argument that is not passed in

function sum (A, b) {

if (b = = = undefined) b = 0;

return a + B;

}

sum (in); Return 3

SUM (1); Return 1, where the second parameter does not pass a value, its value is undefined,

2. Null type (only one value is null, often used to represent an empty object pointer)

• Because it is used to represent an empty object, typeof null = = = ' object '
• Because the undefined value is derived from a null value, NULL = = undefined

3. Boolean type (with two values: true, False)

• Data type conversions using a Boolean ()

String:boolean (' str '); True

Boolean ("); False

Number:boolean (2); True

Boolean (0); False

Object:boolean ({}); True

Null:boolean (NULL); False

Undefined:boolean (Undefined); False

4. String type

• Data type conversions

var myVar = 5;

String (5); Return ' 5 ' to convert any type to string

Myvar.tostring (); Return ' 5 ', undefined with null value without this method

Myvar.tostring (2); Return ' 101 ', returned in binary form

MyVar + = "; Return ' 5 ', automatic conversion with data

5. Number Type

• Floating-point types are best not used in conditional tests due to calculated rounding errors
• Integer 8 binary is not valid in strict mode
• Max Number.MAX_VALUE, Min number.min_value
• Positive Infinity number.positive_infinity, negative infinity number.negative_infinity, function isfinite (value) is used to determine whether a value is between positive infinity and negative infinity
• NaN represents a value that is not a value, and the function isNaN (value) is used to determine whether a value is ' not numeric ' after a data type conversion
• Data type conversion number (), parseint (), parsefloat ()

Number (): You can convert any type of data, and the following expression returns True

Number (undefined) = = = NaN;

Number (NULL) = = = 0;

Number (' 34 ') = = = 34;

Number (' 34df ') = = = NaN;

Number (') = = = 0;

Number (true) = = = 1;

Number (false) = = = 0;

Number ({}) = = = NaN;

parseint (): Primarily used to convert a string to an integral type, the second parameter can be specified as a binary

parseint (' 12df ') = = = 12

parseint (' 2.4dd ') = = = 2

parseint (") = = = NaN

parseint (' 10 ', 10) = = = 10

parseint (' 10 ', 2) = = = 2

Parsefloat (): Used to convert a string to floating-point data, the rule is similar to parseint, but there is no second parameter but can only be converted to 10 floating-point numbers

JavaScript Advanced Programming Series-Basic data types