How to check if an object has a property or key(Javascript).
Hello everyone, today in this post I’m going to share with you some tips to check if a property exists in your javascript object, this also apply for the keys.
hasOwnProperty
The Object data type in javascript allows you to store data structurally as a key/value.
Object have a certain number of properties that come from this data strcuture among which we hasOwnProperty, which is a method that indicates if an object has a specific property.
const student = {
firstName: 'Samy',
lastName: 'Barba',
}
console.log(student.hasOwnProperty('firstName')); // true
console.log(student.hasOwnProperty('lastName')); // true
console.log(student.hasOwnProperty('age')); // false
The function returns a boolean(true/false), so if the property exists we get the value true, even if the property value null or undefined.
It is important to note that the function only checks the properties of our object, not the inherited ones. For example we know that the object has the method toString, so if we try to do:
console.log(student.hasOwnProperty('toString')); // false
we will get false as a response, it does not correspond to a property proper to our object.
in
one of the easiest ways to test the existence of a property is to use the in operator, its use is simple
const student = {
firstName: 'Samy',
lastName: 'Barba',
}
console.log('firstName' in student); // true
console.log('age' in student); // false
I prefer this way because its syntax is short.
There is a big difference to note is that the in operator unlike the hasOwnProperty method allows to detect the own properties and also those inherited by the object.
console.log('firstName' in student); // true
console.log('toString' in student); // true
undefined check
Trying to access an object’s property returns the value of this property, so what happens if it doesn’t exist? In this case we get an undefined, so we can use this value to check if our object has the property or not.
const student = {
firstName: 'Samy',
lastName: 'Barba',
}
console.log(student.firstName); // Samy
console.log(student.age); // undefined
In javascript the values undefined and null correspond to the value false in boolean.
one thing to keep in mind is that in case if the property exists and the value of this property is undefined we will get undefined but yet the object has the property, in most cases we use this method when we are sure that the property can not have as value undefined.
const woman = {
code: undefined,
}
console.log(woman.code); // undefined
Reflect
Reflect is a built-in object that provides methods for interceptable JavaScript operations. The methods are the same as those of proxy handlers. Reflect is not a function object, so it’s not constructible. Reflect MDN
class Reflection has a static has method which takes two arguments, the first one is the object and the second one is the property we want to check the existence of in our object. It return a boolean indicating whether or not the target has the property.
const woman = {
code: undefined,
}
console.log(Reflect.has(woman, 'code')); // true
console.log(Reflect.has(woman, 'power')); // false
In conclusion, we can say that there are mainly four ways to check the existence of a property or key: the hasOwnProperty method, the in operator, the undefined check by accessing the property and finally the has method of the Reflect class. Their use depends on the context of the different constraints. Which one did you like the most and which one you will use in your next code? Do you know any other way? If yes, share it with us in the comments section.
See you in a new post. Thanks