Study notes 7

About js Some of our studies , I also touched a little before , But I don't understand it very well, and I haven't used it much recently , Recently, I watched the video shared by my sister , I learned something new , Simply take notes .

About variables

JavaScript Variables in usually pass var Keyword to declare ：

var name="jun";
var age="19";

var name="jun",age="19";

Besides ,JavaScript Different data types can be stored in the same variable ：

var school = “jxnu”;
school = 123;

in addition ,JavaScript You can use it directly without declaring variables , for example ：

var text1="i am a student";
text2=text1+"from china";

javascript When encountering undeclared variables , It will automatically create a global variable , For good usage , Variables should be declared .

in addition , Variable declarations should follow the following rules ,

The first letter of the variable can be upper and lower case English letters , Underline or dollars “$” Symbol
The remaining letters can be underlined , Case letters , Any number or dollar "$" Symbol
The name of a variable cannot be a reserved word or keyword .

Here are some illegal The variable name of ：

var 1abc; // The beginning of the number 
var hdj“ksjh; // Single quotation marks 
var false; //false Is the key word 

Function objects and function expressions

Function declaration ：function  The name of the function  ( Parameters ： Optional ){  The body of the function  }

Function expression ：function  The name of the function （ Optional ）( Parameters ： Optional ){  The body of the function  }

function foo(){} //  Statement , Because it's part of the program 
var bar = function foo(){}; //  expression , Because it's part of an assignment expression 

new function bar(){}; //  expression , Because it is new expression 

(function(){
 function bar(){} //  Statement , Because it's part of the function body 
})();

If function foo(){} As part of an assignment expression , So it's a function expression

If function foo(){} Contained in a function , Or at the top of the program , So it's a function declaration .

If function foo(){} As part of an assignment expression , So it's a function expression

If function foo(){} Contained in a function , Or at the top of the program , So it's a function declaration .

alert(fn());

function fn() {
 return 'Hello world!';
}

Name functions and expressions

Mention named function expressions , It has to have a name , The previous example var bar = function foo(){}; Is a valid named function expression , But one thing to remember ： This name is only valid in the newly defined function scope .

var f = function foo(){
 return typeof foo; // function --->foo Is valid in internal scope 
};
// foo Used externally is invisible 
typeof foo; // "undefined"
f(); // "function"

call（）,apply（） Method

JavaScript  Pass through call perhaps apply To call a method instead of another object , Change the object context of a function from the initial context to  thisObj  New object specified .

Simply put, change the context of function execution , This is the most basic usage . The basic difference between the two methods is that the parameters are different .

call(obj,arg1,arg2,arg3); call The first parameter is passed to the object , It can be null. Parameters are passed values separated by commas , Parameters can be of any type .
apply(obj,[arg1,arg2,arg3]); apply The first parameter is passed to the object , Parameters can be arrays or arguments  object .
for example ：

function sum(num1, num2){
 return num1 + num2;
}
function callSum1(num1, num2){
 return sum.apply(this, arguments); //  Pass in arguments  object 
}
function callSum2(num1, num2){
 return sum.apply(this, [num1, num2]); //  Pass in array 
}
alert(callSum1(10,10)); //20
alert(callSum2(10,10)); //20

In addition, this week about js And while,if sentence ,arguement Detailed explanation , Basic types , Reference type . and break and continue sentence , I did some related questions .