Revisit the basics of JavaScript (i)

Types of JavaScript

Like the java\c language, the basic type contains (numbers, objects, functions, Booleans, strings), whereas in JS, because of the particularity of null and undefined, the function is also the particularity of the object, so JS data type contains:

Number, String, Boolean, symbol (symbols, new version sixth), Object "Function, Array, Date, RegExp, Math", Null, Undefined

about the number object for JavaScript

JS numbers do not distinguish between integers and floating point numbers, all of which are represented by floating-point number, especially in operations

01 + 0.2 = 0.3000000000000004

　　

Note: The reason is that the IEEE 754 standard is used to calculate, and not only JS is so, where this law is so.

NaN "non-numeric not a number"

Used to indicate that an operand that would have returned a numeric value did not return a numeric value (guaranteed program is not abnormal), js, 0/0 = NaN

Nan = = = Nan//false nan is not equal to any number including itself. NAN/10//nan       nan with any number of operations returns Nan

　　

Methods for numeric conversions:

Number (), parseint (), Parsefloat () method:

Number () Method:

• For various types of conversions

Number (false); 0Number (TRUE); 1Number ("011"); 11Number ("); 0  If NULL, the return value is 0Number (undefined), and//nan  if it is undefined, returns NaN

　　

Pareseint () Method:

• Converts a string to an integral type, and the second parameter represents the base (binary) of the number represented by the string

parseint (' 456 ', 10); 456parseInt ("Hello", 10); NaN

　　

The parseint () method, in the process of converting numeric values, is more concerned with whether or not to conform to the numeric pattern, he ignores the space before the character, finds the first non-whitespace character, and returns Nan if it is not a character or minus sign.

Parsefloat () Method:

parsefloat (' 22.34.5 '); 22.35parseFloat (' 1234blued '); 1234parseFloat (' 0908.5 '); 908.5

　　

The Parsefloat () method is consistent with the parseint (), the difference being that parsefloat () only parses decimal integers, so there is no back base

and the function attempts to parse the characters in the string one after the other, until a character that cannot be parsed into a number is encountered, and then returns a number that consists of all numeric characters before the character. parseInt() parseFloat() Use the operator "+" to convert a string to a number, as long as the string contains a character that cannot be parsed into a number, NaN the string is converted to

string Object about JavaScript

A string in JavaScript is a series of Unicode characters . More precisely, they are a sequence of UTF-16 encoding units, each of which is represented by a 16-bit binary number. Each Unicode character is represented by one or two encoding units.

var text = ' Hello '; Console.log (text.length); 5

　　

The Length property in the string indicates the number of characters in the string

Built-in methods for string objects:

Method	Description
CharAt ()	Returns the character at the established position
charCodeAt ()	Returns the Unicode encoding of the character at the established position
Concat ()	Connection string
IndexOf ()	Retrieves the string position, where it can be set to start retrieving, not found return-1, case sensitive
LastIndexOf ()	Consistent with indexof (), just starting with the last one.
ToString ()	return string
Replace ()	Used to replace some characters with some other character var str= "Visit microsoft!" ; document.write (Str.replace (/microsoft/, "W3school"))
Split ()	Split into a string array
Match ()	Retrieves the specified value within a string, or finds a match for one or more regular expressions

Http://www.w3school.com.cn/jsref/jsref_obj_string.asp

Other methods refer to the links above

Revisit the basics of JavaScript (i)