JavaScript basic knowledge must know!!!

>>>js Basic Knowledge

First, JS role

1, form verification, reduce the pressure on the service side;

2. Add a page animation effect

3. Dynamically change the content of the page

※4, Ajax network requests

II. Basic Structure

<script type= "Text/javascript" >

Alert ("hahaha");

</script>

Three, three ways to use JS

1, directly in the HTML tag, using the event properties, invoke the JS code:

<button onclick= "alert (' Dot me! ‘);" > Point me! </button>

2. In any position on the page, use the script tag to insert the JS code.

<script type= "Text/javascript" >

Alert ("hahaha");

</script>

3, the introduction of external JS file:

<script src= "Js/01.js" type= "Text/javascript" ></script>

[ precautions ]

The ①JS code can be used anywhere on the page, but the placement is different, which will affect the order of JS execution.

② introduction of External JS in the script tag, no longer contain any JS code.

Iv. variables in JS

1. Declaration of variables

var num = 1; variables declared with Var, which are local variables, are only active at the current

num = "hahaha"; variables declared without VAR, default to global variables, available in the entire JS file

var a=1,b,c=2; Declare multiple statements using one line of code. where B is undefined

[Considerations for variable declarations in JS]

The keyword that declares a variable in ①js has only one Var, the type of the variable, depending on the value assigned;

If the declaration is an assignment, then the undefined type.

The same variable in ②js, can be modified in multiple assignments, the data type;

var num1=1;

num = "string";

the ③ variable can be declared with Var, or it can be directly assigned;

[difference] Use VAR to declare the scope to be a local variable.

④ in JS, a variable can be used multiple times VAR declaration, the subsequent declaration is equivalent to direct assignment, without any effect;

⑤JS variables are case-sensitive, uppercase and lowercase are not a variable;

2, JS data type:

Undefined: Variables declared with VAR, but not assigned

Null: Represents an empty reference

Boolean: TRUE or False

Number: Numeric type, including integral and floating-point types

String: Strings

Object: Objects

3. Common numerical functions

① IsNaN: Used to detect is a variable, is not a non-numeric (not a number);

isNaN when detected, the number function is called first, attempting to convert the variable to a numeric type, or Nan if the final result can be converted to a numeric value.

② number function: Used to convert various data types to numeric types

>>>undefined: Cannot convert, return nan;

>>>null: converted to 0;

>>>boolean:true converted to 1,false to 0;

>>> string:

If the string is a pure numeric string, it can be converted, "123"-->123

If the string contains non-numeric characters, it cannot be converted, "123a"-->nan

If it is an empty string, it is converted to 0, "-->0" "-->0

③ parseint (): Converting a string to a numeric type

>>> if it is an empty string, cannot go, ""-->nan

>>> if it is a pure numeric type string, it can be converted, and the decimal point is left out, not reserved, "123"-->123 "123.9"-->123

>>> If the string contains non-numeric characters, the integer preceding the non-numeric character is converted, "123a"-->123 "A123"-->nan

④ parsefloat (): The conversion mechanism is the same as in Java.

The difference is that when converting a numeric string, the decimal point can be preserved if the string is a decimal, "123.5"-->123.5 "123"-->123

⑤ typeof (): detects the data type of a variable.

String->string numeric->number True/false->boolean

->undefined object/null->object function not defined->function

Five, JS commonly used in the input and output statements

1.alert (): pop-up window output

2,prompt (): pop-up window input

Accept two-part parameter: ① input prompt content; ② The default text of the input box. (both parts can be omitted)

The input content is a string by default.

3,document.write("

Print on the browser screen.

4,console.log("hahaha");

Browser console printing.

The operators in the six, JS

1, Division sign: whether the symbol on both sides is an integer or a decimal, except after the end will be in accordance with the actual results to retain decimals ;

22/10-2.2

2, = = =: The data, type and value must be the same on both sides of the equals sign. Returns false directly if the type is different

= =: Only judge the data on both sides, the value is equal, do not care whether the two sides of the equation is the same data type

！ =: Unequal! = =: not congruent

3, &, | Bitwise operations can only be performed, if the two sides are not numeric types, will be converted to numeric types and then operations;

&&, | | Perform logical operations

4. The level of operation of the operators at all levels:

>>>js branching and looping

First, if judgment

1, JS in the True and false judgment:

①boolean type: True for True, false for false;

② value type: 0 is false, not 0 is true;

③ String Type: "" is false, non-empty string is true;

④null/undefined/nan: All is false;

⑤object: All is true.

2. If judgment:

Second, the circulation

1. Switch

Various data types can be placed in the () of the switch structure:

When compared, the "= = =" is used to determine that the data type is exactly equal

JS The difference between switch and switch in Java:

Java in the switch can not judge the interval, and JS in the switch to determine the interval ↓↓↓

2, Do-while

do{

}while (FALSE);

3. For loop

for (Var i=0;i<100;i++) {

}

4, Example: Enter a number, determine whether it is a positive integer, if it is not a positive integer, prompt input error, please re-enter, if it is a positive integer, invert the output of this number.

JavaScript basic knowledge must know!!!