Javascript determines whether there is a certain attribute instance code summary in the object

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

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