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Implementation and use of js constructors, index arrays and attributes_javascript skills
Implementation and use of js constructors, index arrays and attributes_javascript skills
<script>
function p(){
var len=arguments.length;
for(var i=0;i<len;i++){
document.write(arguments[i]+"<br/>");
}
}
function Myclass(x,y){
this.x=x;
this.y=y;
this.show=function(){
return this.x+this.y;
}
}
var m1=new Myclass(1,2);
var m2=new Myclass(3,4);
p(m1.show(),m2.show());
</script>Existing problems
1. Since all instances copy entities defined by the same method, efficiency (low memory efficiency and low execution efficiency) can be solved through prototype inheritance
2. The inability to control access to attribute values (private, public) can be solved by closure
The operand of attribute access is not a variable but a reference to the object
Processing of reading only the integer part of a value
Math[this<0?'celling':'floor'](this);
Associative array
Associative arrays must be implemented through objects in js
Basic operations: getting values through keys, setting elements, deleting elements
<script>
var map={x:3,y:4};
p(map.x);
delete map.x; //true
p(map.x); //undefined 对不存在的元素进行访问结果是undefined ,由于可以显示地将值设置为undefined ，因此无法通过将值与undefined比较来判断值是否存在 ，可以通过for in进行枚举
a='undefined';
p(a);//undefined
p(typeof map.x==a); //true
</script>Points that should be noted as an associative array
<script>
function Myclass(x,y){
this.x=x;
this.y=y;
}
Myclass.prototype.z=5;
var obj=new Myclass(1,2);
for(var key in obj){
p(key+":"+obj[key]); //会枚举出通过原型继承来的属性
}
//x:1 y:2 z:5
delete obj.x;//true
p(obj.x); //undefined
p(obj.z); //5
//通过原型继承来的属性 无法被delete删除
delete obj.z; //true
p(obj.z);//5
//在将对象作为关联数组使用时,通常都会使用字面量来创建，即使视图通过使用空的对象字面量来创建一个没有元素的关联数组，也仍会从Object类中继承原型的属性
p('toString' in obj); //true
var obj1={};
p('toString' in obj1);//true
//通过 for in枚举
p(obj1.length); //undefined
for(var k in obj1){
p(obj1[k]);
}
//没有元素 被枚举出来 这是由于enumerable属性的缘故
//通过hasOwnProperty来判断 是本身的属性还是通过 参与原型继承而来的属性
var map={};
p(map.hasOwnProperty('toString')); //false
map['toString']=1;
p(map.hasOwnProperty('toString')); //true
delete map['toString'] ;
p(map.hasOwnProperty('toString'));//false
</script>Properties of properties
The properties of objects also have some properties
The following table summarizes the attributes defined in ECMAScript 5th Edition. The attribute value is designated as the value attribute
Form 1
|
Meaning |
||||||||||||||||||||||
|
writable |
You can overwrite the value of the attribute |
||||||||||||||||||||||
|
enumerable |
can be enumerated through for in |
||||||||||||||||||||||
|
configurable
|
|
||||||||||||||||||||||
|
get |
Getter function that can specify attribute value
<script>
var obj={x:2,y:3};
Object.preventExtensions(obj);
//无法新增属性
obj.z=4;
p(Object.keys(obj));//x,y
//可以删除属性
delete obj.y;
p(Object.keys(obj)); //x
//可以更改属性值
obj.x=20;
p(obj.x); //20
//Object.seal例子 将属性的configurable设置为假
var obj={x:2,y:3};
Object.seal(obj);
//无法新增 也无法删除
obj.z=3;
p(Object.keys(obj)); //x,y
delete obj.x; //false
p(Object.keys(obj));//x,y
//可以改变 属性值
obj.x=20;
p(obj.x);//20
//Object.freeze例子 将属性的writable设置为假
var obj={x:2,y:3};
Object.freeze(obj);
//无法新增 也无法删除，也无法改变属性值
obj.z=3;
p(Object.keys(obj)); //x,y
delete obj.x;
p(Object.keys(obj));//x,y
//可以改变 属性值
obj.x=20;
p(obj.x);//20
</script>Copy after login
set |
|
Setter function that can specify attribute values
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