js data structure and the data base entry of the converter 01 javascript
js composition (ECMAScript, BOM, the DOM)
js the print statement:
Data type
variable
Operators
data type conversion
js three special values in
the composition of the js (ECMAScript, BOM , the DOM)
①ECMAScript: ECMAScript is a standard that specifies the syntax types, statements, keywords, reserved child, operator, object. (Equivalent law);
②BOM (Browser Object Model): browser window to access and manipulate operations;
③DOM (Document Object Model): DOM entire page is mapped to a multi-node structure. HTML pages are composed of some type of nodes, which in turn contain different types of data;
JS print statement:
①alert () pop pop-up blocking program execution;
②document.write () page printed page, affect the layout, in some cases cover page;
③console.log (); printed to the console;
④prompt () print can enter dialogue popups (dialog box);
⑤confirm () pops up a pop-up box OK or cancel dialog;
data type
1 simple (primitive) data types
①string string character string in quotation marks ( "") are quoted character data package ;
②number digital
NaN is numeric type, represents not a number, not the calculation results
③Boolean Boolean to false to true
④undefind unassigned
⑤null empty
ES6 are new types:
⑥symbol
complex (object) data types
①object objects wrapped with {}
②array array with [] wrapped
③function function
3. data type detection typeof
operation type return values are:
string, string
boolean, Boolean (to true / to false)
number, digital (including NaN, isNaN () is used to determine whether the data NaN3)
undefined, undefined
object, the object
function function
variables
created variables:
statement: var,
assignment operator: =
on variable naming Note:
① must begin with a letter or _ or $
② prefixes beginning with the letter is best to use the data type, can be more semantic
③ keywords and reserved words can not be used as variable names
common keywords: typeof, var, break, continue , return, default , etc.
operators
1. The arithmetic operators
+ plus (+ sign when both sides, as long as the emergence of a character, then that character string splice (connection), the value on both sides, is the value +)
- Save (and implicitly adding character conversion)
* multiplication (and implicitly adding character conversion)
/ addition (addition and character will implicit conversion)
% remainder (and implicitly converted characters are added)
2. The relational operators
<less than
> greater than
<= Less than
> = Greater than or equal to
== equal (Note that a "=" is assigned)
=== strict equality (no implicit type conversion, not only when compared to a comparison value, but also comparison data types)
3. logical operators
|| or (that is, a true is true)
&& and (that is, there is a false, false)
! Non (inverted)
4. increment-decrement
n ++ increases after the first operation, each time plus one
after the first operation ++ n increases, plus one each
n-- reduced after the first operation decrements a
-n reduced after the first operation, each time a Save
The assignment operator
= assignment
+ = cumulative
- like Save =
* = multiplication
/ addition equals =
data type conversion
transfer numeric characters 1.
①parseInt () parse a string, returns an integer, from left to right detection encountered digital conversion, Caught stop numbers, the first one for non-digital converter stops outputting NaN, the decimal point will filter the content that is non-decimal number; the other band can be converted to decimal syntax: the parseInt (NUM, 2 /. 8/16)
②parseFloat ( ) parsing a string and returns a floating point number left to right ,, detection encountered digital conversion, digital conversion Caught encountered non-stop digital output NaN, can recognize the first dot, the second void;
③Math.round () converts strictly, to the nearest integer, not allow any non-numeric, there is the NaN3;
④number () conversion strictly, cast, the decimal point can be identified, does not allow any non-numeric, i.e., appears NaN3
2. digital-to-character
①num.tostring () direct conversion, corresponding to the value added to a quote, may be further converted into other decimal syntax: num.toString (2/8/16)
②num.toFixed () and toString consistent, n may be rounded decimal
3. implicit type conversion rules in the scene if (there is any type of final ture / false value of a Boolean):
① values: two comparison values, that is, as long as 0 ture does not appear;
② characters: two characters compared to non-empty as long (double quotes nothing, including spaces) is the ture;
③: Object are ture ;
④ array: array are ture;
⑤ functions: functions are ture;
⑥underfind, NaN, null: three appear are false;
4.isNaN
JS in three special value
NaN, underfind, null
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