Detecting whether an object contains a key

Use keywords in

The method based on its own attributes and the object to inherit properties exists.

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

Use object hasOwnProperty () method

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

This method can only judge of its own property exists and to inherit property returns false.

Use undefined judge

Own property and inherit property can judge.

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, then the method can not return the desired results, 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

Analyzing in conditional statements

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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