Check if a Key exists in a JavaScript Object

Check if a Key exists in an Object with the in Operator #

To check if a key exists in a JavaScript object, use the in operator, e.g. "key" in myObject. The in operator will return true if the key is in the specified object or its prototype chain.

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const person = {
  name: 'John',
};

console.log('name' in person); // 👉️ true
console.log('age' in person); // 👉️ false

The syntax when using the in operator is: string in object.

The value before the in keyword should be of type string or symbol. Any non symbol value will get automatically coerced to a string.

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const person = {
  0: 'John',
};

console.log('0' in person); // 👉️ true
console.log(0 in person); // 👉️ true

The in operator will coerce the 0 to a string in our second console.log statement.

Check if a Key exists in an Object with Object.hasOwnProperty #

We can use the Object.hasOwnProperty method to check if a key exists in an object, e.g. - myObject.hasOwnProperty('key'). The Object.hasOwnProperty method returns true if the key exists in the object and false otherwise.

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const person = {
  name: 'John',
};

console.log(person.hasOwnProperty('name')); // 👉️ true
console.log(person.hasOwnProperty('age')); // 👉️ false

Check if a Key exists in an Object with Optional Chaining #

We can use the Optional chaining (?.) operator to check if a key exists in an object, e.g. - myObect?.key. If the key exists on the object, the Optional chaining operator will return the key's value, otherwise it returns undefined.

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// Not Supported in IE 6-11
const person = {
  name: 'John',
};

console.log(person?.name); // 👉️ John
console.log(person?.age); // 👉️ undefined

if (person?.name !== undefined) {
  // the key exists on the object
}

In the code example we used the Optional chaining operator to check if the name and age keys exist in the object.

The name key, exists therefore person?.name evaluates to John. The age key doesn't, so the Optional chaining operator returns undefined.

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const person = {
  name: undefined,
};

console.log(person?.name); // 👉️ undefined

if (person?.name !== undefined) {
  // the `name` key exists, but this never runs
}

In the code example, even though the name key exists on the object, our conditional check does not account for the case if it's value is set to undefined.