js method to determine whether there is an attribute

This article mainly shares with you the method of judging whether there are attributes in js. I hope it can help everyone.

Accessing object properties

1. Use the in keyword

This method can determine whether the object's own properties and inherited properties exist.

var o={x:1};  
"x" in o; //true，自有属性存在  "y" in o;
 //false  "toString" in o; 
 //true，是一个继承属性

2. Use the hasOwnProperty() method of the object

This method can only determine whether its own properties exist, and will return false for inherited properties.

var o={x:1};  
o.hasOwnProperty("x"); 　　
 //true，自有属性中有x  o.hasOwnProperty("y"); 　
 　 //false，自有属性中不存在y  o.hasOwnProperty("toString");
 　  //false，这是一个继承属性，但不是自有属性

3. Use undefined to judge

Both own properties and inherited properties can be judged.

var o={x:1};  
o.x!==undefined;
 //true  o.y!==undefined;
  //false  o.toString!==undefined 
  //true

There is a problem with this method. If the value of the attribute is undefined, this method cannot return the desired result, as follows.

var o={x:undefined};  
o.x!==undefined; 
//false，属性存在，但值是undefined  o.y!==undefined; 
//false  o.toString!==undefined //true

4. Directly judge in the conditional statement

var o={};  
if(o.x) o.x+=1; 
//如果x是undefine,null,false," ",0或NaN,它将保持不变

Access object properties

1. Use in Keyword

This method can determine whether the object's own properties and inherited properties exist.

var o={x:1};  
"x" in o; //true，自有属性存在  "y" in o;
 //false  "toString" in o; 
 //true，是一个继承属性

2. Use the hasOwnProperty() method of the object

This method can only determine whether its own properties exist, and will return false for inherited properties.

var o={x:1};  
o.hasOwnProperty("x"); 　
　 //true，自有属性中有x  o.hasOwnProperty("y"); 　　
　  //false，自有属性中不存在y  o.hasOwnProperty("toString");
　   //false，这是一个继承属性，但不是自有属性

3. Use undefined to judge

Both own properties and inherited properties can be judged.

var o={x:1};  
o.x!==undefined; 
//true  o.y!==undefined; 
//false  o.toString!==undefined
 //true

There is a problem with this method. If the value of the attribute is undefined, this method cannot return the desired result, as follows.

var o={x:undefined};  
o.x!==undefined; 
//false，属性存在，但值是undefined  o.y!==undefined; 
//false  o.toString!==undefined //true

4. Directly judge in the conditional statement

var o={};  
if(o.x) o.x+=1;
//如果x是undefine,null,false," ",0或NaN,它将保持不变

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