August 18, 2020
JavaScript check property exists
JavaScript Reference
Check if a JavaScript Object Has a Property
Objects in JavaScript are flexible—you can add or remove properties at runtime. To write defensive code, confirm a property exists before using it. This guide compares the in operator and hasOwnProperty() method.
What You’ll Learn
- How the
inoperator works - When to prefer
hasOwnProperty() - Pros and cons of each approach
Reminder: Prototype chains mean an object may inherit properties you didn’t add explicitly. These checks help avoid unexpected behavior.
Using the in Operator
The expression 'property' in object returns true if the property exists either directly on the object or somewhere along its prototype chain.
const book = { title: "JavaScript" };
console.log('title' in book); // true
console.log('id' in book); // false
console.log('toString' in book); // true (comes from Object.prototype)Using hasOwnProperty()
The hasOwnProperty() method checks only the object’s own properties, ignoring inherited ones.
console.log(book.hasOwnProperty('title')); // true
console.log(book.hasOwnProperty('id')); // false
console.log(book.hasOwnProperty('toString')); // false (inherited)Comparing in vs hasOwnProperty()
| Scenario | Use | Reason |
|---|---|---|
| Need to check prototype chain too | 'property' in object | Covers inherited properties |
| Verify only custom/own keys | object.hasOwnProperty('property') | Ignores prototype pollution |
Putting It Together
const article = Object.create({ slug: 'shared-slug' });
article.title = 'Understanding Property Checks';
console.log('title' in article); // true
console.log(article.hasOwnProperty('title')); // true
console.log('slug' in article); // true (inherited)
console.log(article.hasOwnProperty('slug')); // falseRecommendation: Use hasOwnProperty() to guard user-controlled data, and fall back to in when prototype values should count.
Reusable Utility Helper
export function hasOwn(object, key) {
return Object.prototype.hasOwnProperty.call(object, key);
}
// Example usage
const config = { theme: 'dark' };
console.log(hasOwn(config, 'theme')); // true
console.log(hasOwn(config, 'layout')); // false
JavaScript