JavaScript provides three different ways to check if a property exists in an object or not:
The in operator provides you with the easiest way to check if a property exists in an object. If a property exists in an object, then the in operator returns true. On the other hand, if a property does not exist in the object, then the in operator returns false.
'propertyName' in objectVariable
The following example shows how to check if age and salary properties exist in the employee object:
let employee = {
firstName: "Mohit",
lastName: "Natani",
salary: 100000
};
console.log('age' in employee); //false
console.log('salary' in employee); //true
if('age' in employee){
console.log("age property exists in employee.");
}else{
console.log("age property does not exist in employee.");
}
if('salary' in employee){
console.log("salary property exists in employee.");
}else{
console.log("salary property does exist in employee.");
}
Output
false true age property does not exist in employee. salary property exists in employee.
The best thing about the in operator is that it looks for the inherited properties of an object, whereas the hasOwnProperty() method does not.
The following example shows how to use the in operator to check if the inherited property toString exists in the employee object. The in operator returns true because all objects inherit the toString method from the Object.
let employee = {
firstName: "Mohit",
lastName: "Natani",
salary: 100000
};
console.log('toString' in employee); //true
Object.prototype provides hasOwnProperty() method that returns true if a property exist in an object. If a property does not exist in an object, then the hasOwnProperty() method returns false.
objectVariable.hasOwnProperty('propertyName')
The following example uses the hasOwnProperty() method to check if age and salary properties exist in the employee object.
let employee = {
firstName: "Mohit",
lastName: "Natani",
salary: 100000
};
console.log(employee.hasOwnProperty('age')); //false
console.log(employee.hasOwnProperty('salary')); //true
The hasOwnProperty() method only looks for the property that is owned by the object. It does not look for inherited properties of an object. It returns false if tested on the inherited properties.
The following example shows that the hasOwnProperty() method returns false when tested on the toString property. The reason for this is that the toString is an inherited property.
let employee = {
firstName: "Mohit",
lastName: "Natani",
salary: 100000
};
console.log(employee.hasOwnProperty('toString')); //false
In JavaScript, when you access a property that does not exist in an object, then you get undefined. You can use this information to test if a property exists in an object or not.
let employee = {
firstName: "Mohit",
lastName: "Natani",
salary: 100000
};
if(employee.age === undefined){
console.log("age property does not exist in employee.");
}else{
console.log("age property exists in employee.");
}
if(employee.salary === undefined){
console.log("salary property does not exist in employee.");
}else{
console.log("salary property exists in employee.");
}
Output
age property does not exist in employee. salary property exists in employee.
You cannot rely on this way of checking because if a property has an undefined value, in that case, you will get an incorrect result.
let employee = {
firstName: "Mohit",
lastName: "Natani",
salary: undefined
};
if(employee.salary === undefined){
console.log("salary property does not exist in employee.");
}else{
console.log("salary property exists in employee.");
}
Output
salary property does not exist in employee.
In this example, the salary property exists in the employee object and has an undefined value. So, when compared with undefined gave true, which is not expected.
Note: To learn about undefined data type in JavaScript, visit What is undefined in JavaScript?
