JavaScript self study notes of must see articles
- 2021-07-01 06:05:19
- OfStack
0-Determine whether variables and parameters are initialized
if(x){} // The variable is initialized or the variable is not empty or the variable is not zero
1-Declare functions that do not need to declare return values, parameter types, or even'end of sentences'; '
function sum(i1,i2){return i1+i2}
2-Declare anonymous functions directly and use them immediately
var f=function(i1,i2){return i1+i2;}; alert(f(1,2));// Ordinary anonymous function
alert(function(i1,i2){return i1+i2;}(3,4));// Direct declaration , Use immediately
There is no class concept in 3-js, so some methods look like classes
function Person(name,age){
this.Name=name;// Dynamically add attributes, similar to C# In dynamic A = new ExpendoObject();
this.Age=age;
this.SayHello=function(){alert('Hello,My name is '+name+' I am '+age+' years old.')};
}
var p1=new Person('lorry',21);
p1.SayHello(); // Image class 1 Sample call
p1.Gender=' Male '; // Dynamic increase of'gender ' ' Attribute
alert(p1.Gender);
4-Array objects are arrays, and you don't have to define arrays in advance
var arr=new Array();
arr[0]=0;
arr[1]=1;
arr[2]=2;
for(var i=0;i<=arr.length-1;i++){
alert(arr[i]);
}
5-Array is an array, also Dictionary, also Stack
var dict=new Array();// As Dictionary Use
dict[' I ']='wo';
dict[' Love ']='ai';
dict[' You ']='ni';
alert(dict[' I ']); // Called by key value
alert(dict. Love ); // Like calling properties 1 Sample call (feature of dynamic language)
for(var k in dict){ //js Traversal in
alert(k); //' I ' , ' Love ' , ' You '--> What is printed out is key
}
for(var k of dict){ //js Traversal in
alert(k); //'wo' , 'ai' , 'ni'--> What is printed out is value
}
var arr = [1,2,3,4,5];//Array Simplified creation method of
var arr = {"lorry":21,"cloud":20};// How to create dictionary style
6-Traverse through all elements that the current page can call
var s=null;
for(var k in document){// The properties of the object are all set in the key Appearing in the form of
s+=k+" ;";
}
alert(s);
7-Use an Array-like subscript operation to get the character at a specified position in the string
var s = 'Hello, world!';
s[0]; // 'H'
s[6]; // ' '
s[12]; // '!'
s[13]; // undefined Indexes that are out of range do not report errors, but 1 Rhythm return undefined
It is important to note that strings are immutable, and if you assign a value to an index of a string, there will be no error, but there will be no effect:
var s = 'Test';
s[0] = 'X';
alert(s); // s Is still 'Test'
8-Upper and Lowercase
var s = 'Hello';
s.toUpperCase(); // Return 'HELLO'
var s = 'Hello';
s.toLowerCase(); // Return 'hello'
9-Searches for the location where the specified string occurs
var s = 'hello, world';
s.indexOf('world'); // Return 7
s.indexOf('World'); // The specified substring was not found, returning -1
10-Gets the substring of the specified index interval of the string
function sum(i1,i2){return i1+i2}
The object of 11-JavaScript is an unordered collection data type, which consists of several key-value pairs
function sum(i1,i2){return i1+i2}
12-To detect whether xiaoming has a 1 attribute, use the in operator:
function sum(i1,i2){return i1+i2}
13-Map
var m = new Map([['Michael', 95], ['Bob', 75], ['Tracy', 85]]);//2 Dimensional array initialization method
m.get('Michael'); // 95
var m = new Map();// Direct initialization 1 Empty Map
m.set('Adam', 67); // Add a new key-value
m.set('Bob', 59);
m.has('Adam'); // Does it exist key 'Adam': true
m.get('Adam'); // 67
m.delete('Adam'); // Delete key 'Adam'
m.get('Adam'); // undefined
var m = new Map([[1, 'x'], [2, 'y'], [3, 'z']]);
for (var n of m) { // Traversal Map
alert(n[1] + '=' + n[0]);
}
14-The built-in forEach method of iterable, which accepts a function and automatically calls it back every iteration.
function sum(i1,i2){return i1+i2}
15-Using the map () method of Array, passing in our own function, we get a new Array as the result:
function sum(i1,i2){return i1+i2}
16-Use map () to convert all the numbers of Array to strings:
function sum(i1,i2){return i1+i2}
17-Use reduce () of Array for cumulative calculation
function sum(i1,i2){return i1+i2}
18-Make a great conversion with reduce (): convert [1, 2, 5, 8, 0] to an integer of 12580
var arr = [1, 2, 5, 8, 0];
alert(arr.reduce(function(x,y){
return x*10+y;
}));
19-Filter out some elements of Array with filter ()
function sum(i1,i2){return i1+i2}
By default, the sort () method of 20-Array converts all elements to String and then sorts them, so...
[10, 20, 1, 2].sort(); // [1, 10, 2, 20]
So if you want to sort by number size, you can write this:
var arr = [];
for (var x = 1; x <= 10; x++) {
arr.push(x);
}
document.write(arr+"<br/>");
document.write(arr.sort(function(x,y){
return x<y?true:false;
}));
To ignore the influence of letter case, you must first convert to uppercase or lowercase
var arr = ['Google', 'apple', 'Microsoft'];
alert(arr.sort(function (s1, s2) {
var x1 = s1.toUpperCase();
var x2 = s2.toUpperCase();
return x1 < x2 ?false:true;
})); // ['apple', 'Google', 'Microsoft']
21-closure (Closure) program structure
Assign the function as a return value to the parameter, and call the parameter to obtain the calculation result
var arr = [];
for(var n=1;n<101;n++){
arr.push(n);
}
function lazy_sum(arr){
var sum = function(){
return arr.reduce(function(x,y){
return x+y;
});
}
return sum;
}
var f = lazy_sum(arr);
alert(f());
The returned function is not executed immediately, but until the f() Before executing
function count() {
var arr = [];
for (var i=1; i<=3; i++) {
arr.push(function () {
return i * i;
});
}
return arr;
}
var results = count(); //results It's stored in 3 A function
var f1 = results[0];
var f2 = results[1];
var f3 = results[2];
f1(); // 16 The returned function references a variable i But it is not executed immediately.
f2(); // 16 Wait until 3 When all functions return, the variables they reference i Has become 4 ,
f3(); // 16 So the final result is 16
*** Remember when returning closures: The return function should not refer to any loop variables or variables that will change later !
③ If 1 What if you have to reference a loop variable?
By recreating the 1 Function that binds the current value of the loop variable with its parameters,
No matter how the loop variable changes later, the value bound to the function parameter remains unchanged:
function count() {
var arr = [];
for (var i=1; i<=3; i++) {
arr.push(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
④ In the absence class Mechanism, only function language, with the help of closures, can encapsulate 1 Private variables
function creat_counter(init){
var n = init||0;
return{
add:function(){
n+=1;
return n;
}
}
}
var c = creat_counter();
alert(c.add());//1
alert(c.add());//2
alert(c.add());//3
*** In the returned object, the 1 Closure that carries local variables n And, variables cannot be accessed at all from external code n .
In other words, a closure is a function that carries state, and its state can be completely hidden from the outside world.
⑤ Utilization Math.pow(x, y) Calculation x^2 Or x^3 //Math.pow(x, y)-->x^y
function make_pow(y){
return function(x){
return Math.pow(x,y);
}
}
var pow2 = make_pow(2)
var pow3 = make_pow(3)
alert(pow2(3))//9
alert(pow3(3))//27
22-Arrow function (currently only supported by firefox)//Parameter =
>
Function body
var f = x => x*x*x
alert(f(3)) //27
23-Generation of Fibonacci Sequence with generator
var f=function(i1,i2){return i1+i2;}; alert(f(1,2));// Ordinary anonymous function
alert(function(i1,i2){return i1+i2;}(3,4));// Direct declaration , Use immediately