JS__ This, arguments, cloning, ternary operator__ Duyi

this

1. Function precompile process this --> window

function test(c) {
    
	//var this = Object.create(test.prototype);
	//{
    
	//__proto__ : test.prototype
	//}
	var a = 123;
	function b() {
    }
}
AO {
    
	arguments : [1],
	this : window,//GO  overall situation 
	c : 1,
	a : undefined,
	b : function () {
    }
}
test(1);
new test();// Implicit this Pointing prototype 

function test() {
    
	console.log(this);
}
test();//window{} -->  overall situation 

2. In the global scope this --> window

this //window{}

3. call/apply You can change the function runtime this Point to
4. obj.func(); func() Inside this Point to obj

var obj = {
    
	a : function () {
    // Method 
		console.log(this.name)
	},
	name : 'abc'
}
obj.a();// Who is calling a Method , Methods this Who is the  this-->obj

test questions ：

1.var name = "222";
var a = {
    
	name : "111",
	say : function () {
    
		console.log(this.name);
	}
}
var fun = a.say;// Function reference 
fun()//222  Functions execute globally  this-->window
a.say()// Called  111
var b = {
    
	name : "333",
	say : function (fun) {
    
		//this --- > b
		fun();// Not being called is global   Go through the precompile process  function () {console.log(this.name);} this --> window
	}
}
b.say(a.say);//222
b.say = a.say;//a.say Copy the function of to b.say Replace with 
b.say();// Called  333

summary : If no one calls it, it means the whole situation

2.var foo = '123';
function print(){
    
	var foo = '456';
	this.foo = "789";
	console.log(foo);
}
print();//this-->window 456
 Variant 1:
var foo = 123;
function print() {
    
	this.foo = 234;//this-->window 
	console.log(foo);// The print is global foo, Overall foo Was changed to 234
}
print();//234
 Variant 2:
var foo = 123;
function print() {
    
	//var this = Object.create(print.prototype) this Don't point to window
	this.foo = 234;
	console.log(foo);// The print is global foo, Overall foo Not changed 
}
new print();//123

3. function  test()  and  new test() What are the results of ？
var a = 5;
function test() {
    
	a = 0;
	alert(a);//0
	alert(this.a);//5
	var a;
	alert(a);//0
}
test();
AO {
    
	a : 0,
	this : window
}
function test() {
    
	var this = {
    
		__proto__ : test.prototype;
	}// There's no a
	a = 0;
	alert(a);//0
	alert(this.a);//undefined
	var a;
	alert(a);//0
}
new test();
AO {
    
	a : 0,
	this : {
    } 
}

Written examination questions

1. precompile ： Function declaration overall promotion 
var x = 1, y = z = 0;
function add(n) {
    
	return n = n + 1;
}
y = add(x);//4
function add(n) {
    
	return n = n + 3;
}
z = add(x);//4

2. Which of the following are javascript Language typeof Possible results ：  AC
//boolean,number,string,undefined,object,function
A.string
B.array
C.object
D.null

3. Look below alert What is the result 
function b(x, y, a) {
    
	arguments[2] = 10;//a
	alert(a);//10
}
b(1, 2, 3);// Every bit of the argument list and every bit of the formal parameter list are mapped to each other 
 If the function body is changed to the following , What's the result ？
a = 10;
alert(arguments[2]);//10

4. Comma operator , Only return to the back one 
var f = {
    
	function f() {
    
		return "1";
	},
	function g() {
    
		return 2;
	}
)();
typeof f;//number

5.
var x = 1;
if(function f() {
    }) {
    //true  Function to expression , In parentheses f External not found 
	x += typeof f;//"undefined"
}
console.log(x);//"1undefined"

6. Which of the following expressions results in true：  AD
A.undefined == null//true NaN Not equal to any number, including itself 
B.undefined === null//false  Not absolutely equal to ,NaN Nor is it absolutely equal to itself 
C.isNaN("100")//false  experience Number Implicit type conversion 
D.parseInt("1a") == 1;//true

{
    } == {
    } //false  An object represents a room , Reference value comparison address , Equal addresses make equal 
var obj1 = obj;// Point to the same room 
obj1 == obj//true

7. Please read the following code , Write the execution result of the following program ：
function print() {
    
	console.log(foo);
	var foo = 2;
	console.log(foo);
	console.log(hello);
}
print();    Execution results ：Error:hello is undefined

8. Please read the following code , Write the execution result of the following program ：
function print() {
    
	var test;// By function Cover 
	test();
	function test() {
    
		console.log(1);
	}
}
print();//1

9. Please read the following code , Write the execution result of the following program ：
function print() {
    
	var x = 1;
	if(x == "1") console.log("One!");// An implicit type conversion occurs 
	if(x === "1")console.log("Two!");// No implicit type conversion occurs 
}
print();//One

10. Please read the following code , Write the execution result of the following program ：
function print() {
    
	var marty = {
    
	 	name : "marty",
	 	printName : function() {
    
	 		console.log(this.name);
	 	}
	}
	var test1 = {
    
		name :"test1"
	};
	var test2 = {
    
		name :"test2"
	};
	var test3 = {
    
		name :"test3"
	};
	test3.printName = marty.printName;
	var printName2 = marty.printName.bind({
    name:123});
	marty.printName.call(test1);//test1
	marty.printName.apply(test2);//test2
	marty.printName();//marty
	printName2();//
	test3.printName();//test3
}
print();

11. Please read the following code , Write the execution result of the following program ：
var bar = {
    a : "002"};//{a : 'a'}
function print() {
    
	bar.a = 'a';
	Object.prototype.b = 'b';
	return function inner() {
    
		console.log(bar.a);//a
		console.log(bar.b);//b
	}
}
print()();// first () Return function inner  the second () Execute function inner

arguments

1. arguments.callee Refer to the function itself

function test() {
    
	console.log(arguments.callee == test);//true
}
test();

The immediate execution function does not have a name , If you want to call recursively , What does a function reference say ？arguments.callee Can solve the problem

var num = (function(n){
    
    if(n == 1){
    
        return 1;
    }
    return n * arguments.callee(n-1);
}(20));

function test() {
    
	console.log(arguments.callee);//test
	function demo() {
    
		console.log(arguments.callee);//demo
	}
	demo();
}
test();

2. func.caller-- func The called environment is ? caller No arguments Properties on , It's the function's own properties
   ES5 An error will be reported in the standard mode

function test() {
    
	demo();
}
function demo() {
    
	demo.caller;//test
}

clone

1. Shallow cloning ： Clones obj1 The attribute change of will be linked obj Change at the same time .

var obj = {
    
	name :  'abc',
	age : 123,
	sex : 'female',
	card : ['visa','unionpay']
}
var obj1 = {
    }
function clone(origin, target) {
    
	var target = target || {
    };// Compatibility 
	for(var prop in origin) {
    
		target[prop] = origin[prop];
	}
	return target;
}
clone(obj, obj1);
obj1.card.push('master');
//obj1.card—->['visa','unionpay','master']
//obj.card—>['visa','unionpay','master']

2. Deep cloning ： Clone the same attributes , But the room is different , Changing one will not change the other .
Analysis link ： Reference values only consider arrays and objects . Circular judgement , When encountering arrays and objects, you should judge .

var obj = {
    
    name : "abc",// Original value 
    age : 123,// Original value 
    card : ['visa', 'master'],// Reference value 
    wife : {
    
        name : "bcd",
        son : {
    
            name : "aaa"
        }
    }
}
var obj1 = {
    
    name : "abc",//obj.name
    age : 123,
    card : [obj.card[0],obj.card[1]],
    wife : {
    
        name : "bcd",
        son : {
    
            name : "aaa"
        }
    }
}
// Traversing objects  for(var prop in obj)  Arrays can also for in, Arrays are special objects  prop Subscript subscript 
// 1. Judge whether it is the original value  typeof()—->object
// 2. Determine whether it's an array or an object ( Reference value ) toString instanceof and constructor There is a problem in the parent-child domain 
// 3. Create the corresponding array or object 
// 4. recursive 

First judge obj The value type of the property , Filter out object type (null Also need to consider ), Use toString Method to distinguish an array from an object , Call the original function recursively .

var obj = {
    
    name : 'abc',
    age : 123,
    gender : 'male',
    card : {
    
         card1 : 'visa',
         card2 : 'master'
    }
};
var obj1 = {
    };
function deepClone(origin, target){
    
  	var target = target || {
    },// Fault tolerance , In case the user doesn't transmit target Parameters 
    toStr = Object.prototype.toString,
    arrStr = "[object Array]";
    for(var prop in origin){
    
    	if(origin.hasOwnProperty(prop)){
    // Judge whether it is your own attribute , Avoid taking attributes on the prototype chain 
    		if(origin[prop] !== 'null' && typeof(origin[prop]) == 'object'){
    // Judge   Absolutely not equal to null,typeof yes object--> Reference value 
	            // if(toStr.call(origin[prop]) == arrStr){// Determine whether it is an object or an array 
	            // target[prop] = [];// Create corresponding arrays and objects 
	            // }else{
    
	            // target[prop] = {};
	            // }
	            target[prop] = (toStr.call(origin[prop]) == arrStr) ? [] : {
    };			
	            deepClone(origin[prop], target[prop]);
        	}else{
    // Judge   Original value   exit 
            	target[prop] = origin[prop];
        	}
    	}	
	}
    return target;
}
deepClone(obj, obj1);
//obj.card.push('abc')
//obj1.card-->["visa","master"]

Ternary operator

conditional ？ yes ： no And it will return the value

        var num = 1 > 0 ? 2 + 2 : 1 + 1;//4
        var num = 1 > 0 ? ("10" > "9" ? 1 : 0) : 2; //0  Inside () Bitwise ratio ASCI code