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1. JavaScript Overview
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JavaScript is one of the world's most popular scripting language , is one of three Web developers must learn the language.
(1) defines the page's HTML content
(2) CSS page layout of the provisions of (landscaping)
(3) conduct a web page JavaScript programming -
A back-end qualified personnel must be proficient in JavaScript.
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JavaScript After inserting HTML pages , by all modern browsers execution
2. History
- ECMAScript It can be understood as a standard of JavaScript
- The latest version has to es6 version, but most browsers just stop supporting es5 code
- Development Environment - line environment, inconsistent versions
- Before you write JavaScript code to be set up to support IDEA ES6 syntax , as shown below
3. Getting Started
3.1 introduces JavaScript way
(1) internal label
(2) introduced from the outside
- The example code
- operation result
3.2 The basic syntax
JavaScript and JavaSE largely similar, but JavaScript strict case-sensitive !
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Variables defined type variable name = variable value;
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Introduction browser F12 (developer debugging tools) features
the most used features page when debugging is: element (ELements), Console (Console), the source code (Sources), network (Network) and so on.
(1) element (Elements): used to view or modify the properties of HTML elements, CSS properties, listen for events, breakpoints (DOM breakpoint: In JavaScript debugging, we often use the breakpoints, debug fact in the DOM structure we can also use breakpoints method, which is the DOM breakpoint (DOM breakpoint))
(2) console (console) : console generally used to perform single-use code, see the JavaScript object, view the debug log information or the exception information .
console.log (score) represents the score in the console print variable browser, the equivalent of System.out.println ();
(3) the source (Sources): This page allows you to view the page's HTML source code files, JavaScript source code, CSS source code, in addition to the most important thing is to debug JavaScript source code, you can add a breakpoint to the JS code and so on.
(4) Network (Network): mainly used for viewing web pages and other header information associated with a network connection.
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Console output
3.3 Data Type
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Basic Type : String (String), number (Number The), Boolean (Boolean), the empty (Null), undefined (Undefined), Symbol (ES6 introduces a new type of raw data representing a unique value).
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Reference Type : Array (Array), the object (Object), the function (Function).
(1) String String
string can be any text enclosed in quotation marks may be used single or double quotation marks , as 'abc' or "ABC"
(2) Digital Number The
JS does not distinguish between integers and decimal
123 123 // Integer
123.1 / / float 123.1
1.123e3 scientific notation @
-99 @ negative
NaN3 // Not A Number
Infinity // indicates infinity
(3) Boolean Boolean
Boolean (logical) can have only two values: true or to false
(. 4 ) null and undefined
null null
undefined undefined
(5) logical operators
&& @ and
|| or //
! // non
(6) comparison operators
=
== equal (not the same type, the same value will be determined to be true)
=== absolutely equal (the same type, the same value, the result true)
Note :
① insist on not using == Compare
② NaN == NaN, this is not all of equal value, including their own
③ can be carried out by determining whether the value is isNaN NaN3
④ float problem: to avoid using a float for operation, accuracy problems exist
(7) Array array
value must be Java objects of the same type, the JS in the object need not be the same type !
Array subscript taken: If the out of range, there will be undefined
(. 8) Object Object
objects are braces , brackets array is the attribute between each separated by a comma , a need to add the last comma
console output
4. Strict check mode
- 'use strict'; strict inspection mode, the prevention of arbitrary JavaScript in the lead to problems arising
and must be written in the first line of JavaScript !
- Local variables is recommended to use let definitions
