Summary of methods to determine whether a certain attribute exists in an object using javascript_javascript skills

There are several ways to detect the presence or absence of attributes in an object.
1. Use the in keyword
This method can determine whether the object's own properties and inherited properties exist.

Copy code The code is as follows:

var o={x:1};
"x" in o; //true, own property exists
"y" in o; //false
"toString" in o; //true, it is an inherited property

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.

Copy code The code is as follows:

var o={x:1};
o.hasOwnProperty("x"); //true, there is x in the own property
o.hasOwnProperty("y"); //false, there is no y in the own property
o.hasOwnProperty(" toString"); //false, this is an inherited property, but not a self-owned property

3. Use undefined to judge
Both self-owned and inherited properties can be used judge.

Copy code The code is as follows:

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

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

Copy code The code is as follows:

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

4. Directly judge in the conditional statement

Copy the code The code is as follows:

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