4 Ways To Check if an Object Key Exists

4 Ways To Check if an Object Key Exists

1. hasOwnProperty

This will return a boolean value depending on whether the object on which you are calling it has a property with given name as an argument e.g. obj.hasOwnProperty('keyname')

Full Example:
class Employee {
    constructor(firstName, lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
  }
}

class Engineer extends Employee {
    constructor(firstName, lastName, qualification) {
        super(firstName, lastName);
        this.qualification = qualification;
    }
}

let eng = new Engineer("John","Doe","BSC")

console.group("hasOwnProperty");
console.log(eng.hasOwnProperty('firstName'))      // Output: true
console.log(eng.hasOwnProperty('qualification'))  // Output: true
console.log(eng.hasOwnProperty('nonexistantkey')) // Output: false
console.groupEnd();

2. Object.keys

Object.keys will return an array of all the keys belonging to the given object.

You can then check the array to see if a particular key exists. It is the longest of all the methods.

class Employee {
    constructor(firstName, lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
  }
}

class Engineer extends Employee {
    constructor(firstName, lastName, qualification) {
        super(firstName, lastName);
        this.qualification = qualification;
    }
}

let eng = new Engineer("John","Doe","BSC")

console.group("iteration");
for (const key of Object.keys(eng)) {
   console.log(key)
   //Perform Other Check ..
}

/* Results:
"firstName"
"lastName"
"qualification"
*/

console.groupEnd();

// Or

Object.keys(eng).filter(x => x.includes("keyname"));

3. in operator

The in operator returns true if the property is in the object

class Employee {
    constructor(firstName, lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
  }
}

class Engineer extends Employee {
    constructor(firstName, lastName, qualification) {
        super(firstName, lastName);
        this.qualification = qualification;
    }
}
let eng = new Engineer("John","Doe","BSC")

console.group("in operator");
console.log('firstName' in eng)      // Output: true
console.log('qualification' in eng)  // Output: true
console.log('nonexistantkey' in eng) // Output: false
console.groupEnd();

4. Reflect.has()

Reflection is the ability of a process to examine, introspect, and modify its own structure and behavior.

Therefore using the Reflect api you can check and/or manipulate an Objects properties, methods and variables.

You can use Reflect.has(targetObject, propertyKey) method to check if the key exists.

#####Full Example

class Employee {
    constructor(firstName, lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
  }
}

class Engineer extends Employee {
    constructor(firstName, lastName, qualification) {
        super(firstName, lastName);
        this.qualification = qualification;
    }
}

let eng = new Engineer("John","Doe","BSC")
console.group("Reflect.has");
console.log(Reflect.has(eng,'firstName'))       // Output: true
console.log(Reflect.has(eng,'qualification'))   // Output: true
console.log(Reflect.has(eng,'nonexistantkey'))  // Output: false
console.groupEnd();

Slán go fóill

Well that's it, each is simple enough and you can choose whichever to do the task.

Feel free to ask questions, comment or contribute below!

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