JavaScript advanced Programming--Chapter III: Basic Concepts

JavaScript Advanced Programming--Chapter III: Basic Conceptsfirst, the grammar

EMCA-262 describes the basic concept of JavaScript implementation by "pseudo-language" called ECMAScript.

JavaScript draws on the syntax of C, is case-sensitive, the identifier begins with a letter, an underscore, or a dollar sign ($) , and the comment can be used // or/ * * /

Strict mode:

ECMAScript 5 introduces strict mode, where indeterminate behavior is handled in strict mode, and strict mode is enabled by adding "use strict" at the top:

function fuc () {    "use strict";
function Body}

This line of code is a compile prompt that tells JavaScript-enabled engines to switch to strict mode.

　　Variable:

　　The type of ECMAScript is loosely typed and can hold any type of data

Declare a variable by using var :

var message,      Num.      STR;

when you omit the var keyword, the global variable is created automatically. 　　

　　Data type:

　　Six types of data
1, Undefined
2, Null
3, Boolean
4. Number
5, String
6, Object (a set of unordered name value pairs)

　　Operator: typeof Note is not a function that can be used for variables or numeric literals. 
　　
　　Undefined:
The undefined type has only one value, that is, a special undefined,
When the variable definition is uninitialized, its value is undefined.

var　　　　test;  typeof test = = "undefined"    //true

　　   Undefined is also returned when performing a typeof operation on an undeclared variable
　　
　　Null:
The value of the null type is NULL, which represents an empty object pointer,
　　　　
The undefined value is derived from a null value.

typeof null = = ' object ';

If a defined variable is intended to be used to hold an object in the future, it is better to initialize the variable to null instead of the other value.
This allows you to know whether a variable has saved a reference to an object as long as it detects whether it is a null value.

　　Boolean:
The Boolean type, with a value of true and false, is case-sensitive.
Any one of the data types to be converted to Boolean simply calls the Transform function Boolean ()

var msg = "Hello"; var msgtobool = Boolean (msg); True

Data type	Convert to True	Convert to False
Boolean	True	False
String	Non-empty string	"" Empty string
Number	Not 0 digits	0 and Nan
Object	Any object	Null
Undefined	Not applicable	Undefined

The control leave statement (if) automatically performs the associated Boolean conversion.
　　
Number:
　　　　Use IEEE754 to represent integers and floating-point values,
　　　　

var intnum =/           / integer var octnum = 020          // octal is not valid in strict mode, throws an error  var hexnum = 0xa          // hexadecimal arithmetic operations are converted to decimal values.

Numerical range number,min_value; Number.MAX_VALUE; Returns inifinity, cannot participate in the operation.

NaN
A non-numeric value (not a number) is a special value that is used in cases where an operand that would have returned a numeric value did not return a numeric value. For example, any value in ECMAScript will return nan in addition to 0.

　　　　function isNaN () to determine whether the parameter is "not a numeric value";

IsNaN (NaN)   //true  IsNaN (//false converted to 10
　　　　
IsNaN ("Blue")//true

numeric conversions

varnum = Parentint ("1234blue");//1234varnum = Parentint ("")//NaNvarnum = Parentint ("0xa")//Tenvarnum = Parentinr (22.5)// Avarnum = Parentint ("070")// Aboutvarnum = Parentint ("70")// -parsing octal ECMAScript3There's a disagreement with ECMAScript 5 .varnum = Parentint ("070")//3 ECMAScriptvarnum = Parentint ("070")//5 ECMAScriptThe second parameter specifies a cardinalityvarnum = Parentint ("AF", 16)//175varnum = Parentint ("AF");//NaN　　　　

Parentfloat () converts only 10 binary. 

　

JavaScript advanced Programming--Chapter III: Basic Concepts