JavaScript Overview of JavaScript history
- 1992 Nombas developed the embedded scripting language for C-minus-minus (c--) (originally bundled in Cenvi software). Rename it scriptease. (The language that the client executes)
- Netscape (Netscape) received Nombas's philosophy, (Brendan Eich) developed a set of Netscape scripting languages in its Navigator livescript 2.0 product. Sun and Netscape are done together. And then the name is JavaScript.
- Microsoft then emulated a JavaScript clone called JScript in its IE3.0 product.
- To unify the three, the ECMA ( European Computer Manufacturing Association) defines the ECMA-262 specification. The International Organization for Standardization (ISO/IEC) also adopted ECMAScript as the standard (iso/iec-16262). Since then, Web browsers have struggled (albeit with varying degrees of success and failure) to ECMAScript as the basis for JAVASCRIPT implementations. ECMAScript is the norm.
ECMAScript
Although ECMAScript is an important standard, it is not the only part of JavaScript, and certainly not the only one that is standardized. In fact, a complete JavaScript implementation is made up of the following 3 different parts:
- Core (ECMAScript)
- Document Object Model (DOM) Documents object models (consolidated js,css,html)
- Browser object models (BOM) Broswer object Model (integrated JS and browser)
- The vast majority of Javascript in development is object-based. It is also object-oriented.
To put it simply, ECMAScript describes the following:
- Grammar
- Type
- Statement
- Key words
- Reserved words
- Operator
- Object (encapsulates inherited polymorphism) object-based language. Use an object.
how JavaScript is introducedwrite code inside the script tag
<script> //write your JS code here </script>
Introduction of additional JS files
<script src= "Myscript.js" ></script>
JavaScript Language SpecificationComments (Comments are the mother of code)
This is a single-line comment/* This is a multiline comment */
Terminator
The statements in JavaScript are terminated with a semicolon (;).
JavaScript Language BasicsVariable Declaration
- The variable name of JavaScript can be composed of _, number, letter, $, and cannot begin with a number.
- declaring variables using var variable names; The format to declare
var name = "Beibei"; var age = 18;
Attention:
Variable names are case-sensitive.
Camel-named rules are recommended.
JavaScript data types
JavaScript has a dynamic type
var x; At this point x is Undefinedvar x = 1; At this point x is the number var x = "Beibei"
Number Type
JavaScript does not differentiate between integral and floating-point types, there is only one numeric type.
var a = 12.34;var b = 20;var c = 123e5; 12300000var d = 123e-5; 0.00123
Common methods:
parseint ("123") //Return 123parseInt ("ABC") //Return Nan,nan property is a special value that represents a non-numeric value. This property is used to indicate that a value is not a number. Parsefloat ("123.456") //Return 123.456
String
var a = "Hello" var b = "World;var c = a + B; Console.log (c); Get HelloWorld
Common methods:
Method |
Description |
Obj.length |
return length |
Obj.trim () |
Remove whitespace |
Obj.trimleft () |
Remove the left margin |
Obj.trimright () |
Remove the blank on the right |
Obj.charat (N) |
Returns the nth character |
Obj.concat (value, ...) |
Stitching |
Obj.indexof (substring, start) |
Sub-sequence position |
Obj.substring (from, to) |
Get sub-sequences by index |
Obj.slice (start, end) |
Slice |
Obj.tolowercase () |
Lowercase |
Obj.touppercase () |
Capital |
Obj.split (delimiter, limit) |
Segmentation |
Splicing strings generally use "+"
Boolean type
The difference between python,true and false is lowercase.
var a = True;var B = false;
Array
Similar to the list in Python.
var a = [123, "ABC"];console.log (A[1]); Output "ABC"
Common methods:
Method |
Description |
Obj.length |
The size of the array |
Obj.push (Ele) |
Trailing append Element |
Obj.pop () |
Gets the trailing element |
Obj.unshift (Ele) |
Head Insert Element |
Obj.shift () |
Removing elements from the head |
Obj.slice () |
Slice |
Obj.reverse () |
Reverse |
Obj.join (seq) |
Concatenate array elements into a string |
Obj.concat (Val, ...) |
Connection array |
Obj.sort () |
Sort |
To iterate over an element in an array:
var a = [Ten, A, 40];for (Var i=0;i<a.length;i++) { console.log (i);}
Object
Similar to (in some respects) a dictionary data type in Python
var a = {"name": "Beibei", "Age": 18};console.log (A.name); Console.log (a["age"]);
Iterate through the contents of the object:
var a = {"name": "Alex", "Age": 18};for (var i in a) { console.log (i, A[i]);}
null and undefined
- Undefined means that when the declared variable is not initialized, the default value of the variable is undefined. There is also the undefined that is returned when the function has no definite return value.
- Null indicates that the value does not exist
Undefined indicates that a variable was declared, but has not been assigned a value. Null declares the variable and the variable is a null value.
Type query
typeof "ABC" //"string" typeof null //"Object" typeof true //"Boolean" typeof 123//"number"
typeof is a unary operator (like ++,--,! ,-the unary operator), is not a function, nor is it a statement.
Operator arithmetic operators
+ - * / % ++ --
comparison Operators
> >= < <= = = = = = = =!==
Attention:
1 = = "1" //true1 = = = "1" //False
logical Operators
&& | | !
Assignment Operators
= += -= *= /=
Process ControlIf-else
var a = 10;if (a > 5) { Console.log ("Yes");} else { Console.log ("No");}
If-else If-else
var a = 10;if (a > 5) { Console.log ("a > 5");} else if (a < 5) { Console.log ("A < 5");} else { Console.log ("a = 5");}
Switch
var day = new Date (). GetDay (), switch (day) {case 0: console.log ("Sunday"); break; Case 1: console.log ("Monday"); Break;default: console.log ("...")}
For
JavaScript for the front-end base