<! DOCTYPE HTML > < HTML > < head > < Meta charset = "UTF-8" > < title > JS knowledge </ title > < Script of the type = "text / JavaScript" src = "index.js" > </ Script > <-! JS reference Analyzing data type; Typ three ways; inline; <a href = "javacscript: alert ( ' Hello');"> </a> embedded; <script type = " text / JavaScript "> Alert ( 'good morning'); </script> External reference; srcitpt type = "text / javascript " src = "index.js"> </ srcitpt> bomb box: alert ( ''); two types of variables defined manner; effect; value assignment or implicitly defined: directly variable assignment display definitions: definition of the variable var keyword js elementary data type numeric type comprising integer and floating point number; divided 1. 2. 3. 4. 5. 6. 1.2 2.5 3.6 4.7 string string type double quotes and single quotation marks value Boolean boolean true true false false two values undefined type undefined empty type (null): used to indicate that a variable is empty (that the variable has been assigned, but Fu is empty) ; object type (Object): array, date and other objects is determined using == data == comparison data alignment must be numeric === match the data type and values + two functions First; when both plus is the value, the addition operation for the second; when the plus side of the string is intended, the stitching type conversion basic data types; the value into a string (toString ( )) the parseInt () to convert the string to an integer parseFloat () string into float -> </ head > < body > <-! <A the href = "javacscript: Alert ( 'Hello') ; "> point I </a> the Button -> </ body > </ HTML >