JavaScript self-study notes (must read)

if(x){} //The variable is initialized or the variable is not empty or the variable is not zero

function sum(i1,i2){return i1+i2}

var f=function(i1,i2){return i1+i2;}; alert(f(1,2));//Ordinary anonymous function
alert(function(i1,i2){return i1+i2;}(3,4));//Declare directly, use immediately

function Person(name,age){

=name;// Dynamically add attributes, similar to Cdynamic A in # = new ExpendoObject();=age;

=function(){alert('Hello,My name is '+name+' I am '+age+' years old.')};

}

var p1=new Person('lorry',21);

(); //Call like a class
='male';    // Dynamically add 'gender' attribute
alert();

var arr=new Array();

arr[0]=0;

arr[1]=1;

arr[2]=2;

for(var i=0;i<=-1;i++){

alert(arr[i]);

}

var dict=new Array();//Use as Dictionary
dict['I']='wo';

dict['like']='ai';

dict['you']='ni';

alert(dict['I']); //Call through key value
alert(dict.like); //Call like calling properties (the characteristic of dynamic language)
 

for(var k in dict){ //Travel in js
 alert(k);  //'I', 'love', 'you'-->what prints out is the key
}

for(var k of dict){ //Travel in js
 alert(k);  //'wo', 'ai', 'ni'-->The printed value is
}

var arr = [1,2,3,4,5];//Simplified creation of Array
var arr = {"lorry":21,"cloud":20};//How to create dictionary style

var s=null;

for(var k in document){//The properties of the object appear in the form of a key
 s+=k+" ;";

}

alert(s);

var s = 'Hello, world!';

s[0]; // 'H'

s[6]; // ' '

s[12];    // '!'

s[13];    // undefined indexes that are out of range will not report an error, but will return undefined
It is important to pay special attention to，Strings are immutable，If you assign a value to an index of a string，There will be no errors，but，No effect either：

var s = 'Test';

s[0] = 'X';

alert(s);  // sStill for'Test'

var s = 'Hello';

();  // Return to 'HELLO'
    

var s = 'Hello';

();  // return'hello'

var s = 'hello, world';

('world'); // Return to 7
('World'); // No specified substring was found, return -1

var s = 'hello, world'

(0, 5);  // Start from index 0 to 5 (excluding 5), return 'hello'
(7);// From index 7 to end, return 'world'

var xiaoming = {

 name: 'Xiao Ming',

 birth: 1990,

 school: 'No.1 Middle School',

 height: 1.70,

 weight: 65,

 score: null//The last key-value pair does not need to be added ',' at the end
};

;   // 'Xiao Ming'
;    // 1990

Access properties is through.Operator completed，But this requires that the attribute name must be a valid variable name。If the attribute name contains special characters，It must be used[]Cover it up：


var xiaohong = {

  name: 'Little Red',

  'middle-school': 'No.1 Middle School'

};

xiaohong['middle-school'];   // 'No.1 Middle School'

xiaohong['name'];  // 'Little Red'
;    // 'Little Red'
; // undefined

'name' in xiaoming;// true

'grade' in xiaoming;// false

***ifinDetermine that an attribute exists，This attribute does not necessarily meanxiaomingof，It may bexiaoming继承得到of：


'toString' in xiaoming;   // true

***To determine whether an attribute isxiaoming自身拥有of，而不是继承得到of，Can be usedhasOwnProperty()method：


('name');    // true

('toString'); // false

var m = new Map([['Michael', 95], ['Bob', 75], ['Tracy', 85]]);//Two-dimensional array initialization method
('Michael'); // 95

 

var m = new Map();// Directly initialize an empty map
('Adam', 67); // Add a new key-value
('Bob', 59);

('Adam');    // Is there a key 'Adam': true
('Adam');    // 67

('Adam'); // Delete key 'Adam'
('Adam');    // undefined

 

var m = new Map([[1, 'x'], [2, 'y'], [3, 'z']]);

for (var n of m) {   // traverse the map
 alert(n[1] + '=' + n[0]);

}

var a = ['A', 'B', 'C'];

(function (element, index, array) {

 // element: Point to the current element value
 // index: Point to the current index
 // array: Point to the Array object itself
 alert(element);

});

    

SetandArraysimilar，butSetNo index，Therefore, the callback function has up to two parameters：


var s = new Set(['A', 'B', 'C']);

(function (element, set) {

 alert(element);

});

 

MapThe callback function parameters arevalue、keyandmapitself：


var m = new Map([[1, 'x'], [2, 'y'], [3, 'z']]);

(function (value, key, map) {

 alert(value);

});

    

var a = ['A', 'B', 'C'];

(function (element) {

 alert(element);

});

var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];

(function(x){

return x*x;

}).forEach(function (element) {

alert(element);// [1, 4, 9, 16, 25, 36, 49, 64, 81]

});

var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];

(String); // ['1', '2', '3', '4', '5', '6', '7', '8', '9']

var arr = [];

for (var x = 1; x <= 100; x++) {

 (x); //Put 1~100 into the array
}

alert((function(x,y){

return x+y;   //Cumulative sum of all objects of arr, return the sum result
}));

var arr = [1, 2, 5, 8, 0];

alert((function(x,y){

return x*10+y;

}));

var arr = [0,1,2,3,4,5,6,7,8,9];

alert((function(x){

return x%2===0;

}));//0,2,4,6,8 //Return true and reserved
    

Put oneArrayDelete the empty string in

var arr = ['A', '', 'B', null, undefined, 'C', ' '];

alert((function (s) {

 return s && (); // Note: There is no trim() method for versions below IE9
})); // ['A', 'B', 'C']

[10, 20, 1, 2].sort(); // [1, 10, 2, 20]

So if you want to sort by number size，Can write this：

var arr = [];

for (var x = 1; x <= 10; x++) {

 (x);

}

(arr+"<br/>");

((function(x,y){

return x<y?true:false;

}));

 

To ignore the case effect of letters，Convert to uppercase or lowercase first


var arr = ['Google', 'apple', 'Microsoft'];

alert((function (s1, s2) {

 var x1 = ();

 var x2 = ();

 return x1 < x2 ?false:true;

})); // ['apple', 'Google', 'Microsoft']

①Assign a function as a return value to a parameter，Call this parameter to obtain the calculation result


var arr = [];

for(var n=1;n<101;n++){

(n);

}

function lazy_sum(arr){

 var sum = function(){

 return (function(x,y){

 return x+y;

 });

 }

 return sum;

}

var f = lazy_sum(arr);

alert(f());

    

②The returned function is not executed immediately，But until it is calledf()Only execute


function count() {

 var arr = [];

 for (var i=1; i<=3; i++) {

 (function () {

 return i * i;

 });

 }

 return arr;

}

var results = count(); //There are 3 functions stored in the results
var f1 = results[0];

var f2 = results[1];

var f3 = results[2];

    

f1(); // 16 The returned function refers to the variable i, but it is not executed immediately.
f2(); // 16 When all 3 functions return, the variable i referenced has become 4.
f3(); // 16 So the final result is 16
***Remember when returning to the closure：Return function does not refer to any loop variables，Or variables that will change later!

 

③What to do if you have to refer to loop variables？

 The method is to create another function，Use the parameters of this function to bind the current value of the loop variable，

 Regardless of how the loop variable is subsequently changed，The value bound to the function parameter remains unchanged：


function count() {

 var arr = [];

 for (var i=1; i<=3; i++) {

 (function(n){

 return function(){

 return n*n;

 }

 }(i));

 }

 return arr;

}

 

var results = count();

var f1 = results[0];

var f2 = results[1];

var f3 = results[2];

 

alert(f1()); // 1

alert(f2()); // 4

alert(f3()); // 9

 

④In noclassmechanism，Only in the language of functions，With the help of closure，Can encapsulate a private variable


function creat_counter(init){

 var n = init||0;

 return{

 add:function(){

 n+=1;

 return n;

 }

 }

}

    

var c = creat_counter();

alert(());//1

alert(());//2

alert(());//3

***In the returned object，Implemented a closure，The closure carries local variablesn，and，Variables are simply inaccessible from external coden。

in other words，Closure is a function that carries state，and它的状态可以完全对外隐藏起来。

 

⑤use(x, y)calculatex^2orx^3 //(x, y)-->x^y


function make_pow(y){

 return function(x){

 return (x,y);

 }

}

    

var pow2 = make_pow(2)

var pow3 = make_pow(3)

    

alert(pow2(3))//9

alert(pow3(3))//27

var f = x => x*x*x

alert(f(3)) //27

function* fib(max){

 var t,a=0,b=1,n=1;

 while(n<=max){

 yield a;

 t=a+b;

 a = b;

 b = t;

 n++;

 }

 return a;

}

for (var x of fib(10)) {//Iterate the generator object with for ... of loop
  (x+'&nbsp'); // Output 0, 1, 1, 2, 3 in turn
}

 

usegeneratorGenerates an autoincrementID(No global variables required)

function* next_id(){

for(var x = 1; x < 100; yield x++ );

}

var g = next_id();

alert(().value);//1

alert(().value);//2

alert(().value);//3