Basic syntax for JavaScript

1.JavaScript relationship to DOM:
The browser has the function of rendering HTML code, the HTML source in memory to form a DOM object, is the document object.
Inside the browser there is a JavaScript interpreter/actuator/engine.
Write a JavaScript code in HTML that the JavaScript is executed by the engine, and the result is the operation of the DOM.
And the result of the operation of the DOM is that we often see the special effects, tablets floating, text color.
The host of JavaScript is not just a browser, it can also be a server.

Learning JavaScript mainly distinguishes between:
(1). The syntax of the JavaScript language itself.
(2). Dom Object (body,div,p node tree as an object)

(3). BOM objects (place the browser's address bar, history, DOM, etc.) in an object.

2.

3. Naming conventions:
The variable name of JavaScript can be composed of _, number, letter, $, and the number cannot begin.
Declaring a variable is declared with the VAR variable name.
var a = 34;
var B = 45;
alert (A+B);
Console.log (A,B,A+B);
var $c = ' World ';
alert ($c);
var $= ' jquery ';
Alert ($);

Note: Variables in JavaScript need to be case sensitive. STR and STR are not a variable.
Note: Without Var, the global variables will be polluted.

Variable type of 4.//JS
var a = 23;//integer type
var B = 3.14;//Floating-point number type
var c = "Hello";//String type
var d = ' world ';
var e = true; BOOL Type
var f = null;//No object data
var g = undefined;//No original data
var h = {name: ' Lisi ', age:20};//associative array, object type
Console.log (H.name);
Console.log (h[' name ');

var arr=[' A ', 3, ' Hello ', true];//array always starts incrementing from 0,1,2 ...
Console.log (arr);
Alert (arr);
Console.log (arr[2]);

concatenation operator "+" in 5.//js
Console.log (2+3);
Console.log (' hello ' + ' + ' world ');
Console.log (2+3+4+ ' haha ' +5+6);//9haha56, once the illegal numbers are encountered, the following are directly spliced.

The logical operation in JS, which returns the first value that can judge the result of an expression.
var a = false;
var B = 6;
var C = true;
var d = (a| | b| | c);
var e = (a| | c);
Console.log (d);//6
Console.log (e);//true
Console.log (a&&b);//flase

Control structure in 6.JavaScript:

var arr = [' Zhao ', ' money ', ' sun ', ' Li '];
for (var i = 0; i < arr.length; i++) {
Console.log (Arr[i]);
}
var obj = {
Name: ' Lisi ',
Age:29,
Area: ' BJ '
};
For (var k in obj) {
Console.log (k + ' ~ ' +obj[k]+ ' ~ ' +obj[' K ']);
}
/* Run Results
name~lisi~undefined
age~29~undefined
area~bj~undefined
*/

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Basic syntax for JavaScript