js to detect whether a property exists in an object

[size=large]
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, own property exists
"y" in o;            //false
"toString" in o; //true, is an inherited property

2. Use the hasOwnProperty() method of the object.

This method can only determine whether the own property exists, and returns false for inherited properties.

var o={x:1};
o.hasOwnProperty("x"); //true, there is x in its own property
o.hasOwnProperty("y"); //false, y does not exist in its own property
o.hasOwnProperty("toString"); //false, this is an inherited property, but not an own property

3. Use undefined to judge both

own attributes and inherited attributes.

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 property is undefined, the method cannot return the desired result, as follows.

var o={x:undefined};
ox!==undefined; //false, the property exists, but the value is undefined
o.y!==undefined;        //false
o.toString!==undefined  //true
 

4. Direct judgment in the conditional statement

var o={};
if(ox) o.x+=1; //If x is undefined,null,false," ",0 or NaN, it will remain unchanged

[/size]