JavaScript Basics (third time)

Variable

Person=null; Empty variables

Carname= "Volvo"; Assign to an undeclared variable, it will become a global variable, even inside the function

Window.carnam; All data variables belong to the Window object

Character accessing a string: Using the location index

var carname = ' Volvo XC60 ';

var character = carname[7];

var y = new String ("John"); Do not use this method to declare variables

Data type

typeof [1,2,3,4]; Return object

typeof null; Return object

typeof new Date (); Return object

typeof new Array (); Return object

typeof NaN; return number

var answer; Console.log (typeof answer); Back to undefined

typeof undefined; Undefined

typeof null; Object

NULL = = undefined//False

NULL = = undefined//True

typeof function () {}//return function

typeof ABC; Back to "undefined"

5 Types of data:

String

Number

Boolean

Object

function

3 Types of objects:

Ojbect

Date

Array

3 types of data that do not contain any values:

Null

Undefined

Number (""); Returns 0

Number (""); Returns 0

Number ("99 88"); Return NaN

JS exception

try{

var X=document.getelementbyid ("Demo"). Value;

if (x== "") throw "empty";

}catch (Err) {

var Y=document.getelementbyid ("mess");

Y.innerhtml= "Error:" + err;

}

Javascript:void (0) means dead link , click No response

JS function

declaration by expression :

var x = function (A, b) {return a*b;}

functions can be called before a declaration : only function mode

MyFunction (5);

function MyFunction (y) {

return y*y;

}

self-calling functions :

(function () {

var x = "Hello";

})();

Functions can be used as a value:

function func (x, y) {

return x*y;

}

var a = func (4,3);

var B = func (4,3) + 5;

Default Parameters :

function myFunction (x, y) {

y = y | | 0;

}

Arguments object:

var x = SumAll (1, 123, 500, 115, 44, 88);

function SumAll () {

var i, sum = 0;
for (i = 0; i < arguments.length; i++) {
Sum + = Arguments[i];
}
return sum;

}

Call/apply: Calling function

function MyFunction (A, b) {

return a * b;

}

Myfunction.call (MyObject, 10, 2);

MyArray = [10,2]; Myfunction.apply (MyObject, MyArray);

;

JavaScript Basics (third time)