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  3. Check If a Value Is an Object in JavaScript or Node.js

Check If a Value Is an Object in JavaScript or Node.js

Sometimes you need to check for the type of a variable before moving on with any processing. For example, you may want to validate a value to be an object before merging its properties with another object.

This tutorial shows you how to confidently check whether a given value is a JavaScript object.

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Detect Whether a Value Is an Object Using JavaScript

Intuitively, you may think about using JavaScript’s typeof operator when checking whether a given value is an object. But there’s a drawback using typeof: JavaScript treats different types as objects.

For example: an object is an object, an array is an object, a date is an object, null is an object. That means, using typeof requires checks for each special object data type:

typeof { name: 'Future Studio' }  
// 'object'

typeof new Date()  
// 'object'

typeof null  
// 'object'

typeof [1, 2]  
// 'object'

We recommend using the Object.prototype.toString method. This method returns the stringified type identifier. By type identifier, we mean Object, Array, Date, and so on. And the stringified version has a format of [object <type>]. Here are some examples:

Object.prototype.toString.call([1, 2, 3])  
// '[object Array]'

Object.prototype.toString.call('Future Studio')  
// '[object String]'

Object.prototype.toString.call({ name: 'Future Studio' })  
// '[object Object]'

We can now compare the output of Object.prototype.toString against the desired object type. Let’s create a reusable isObject utility function from the observations:

/**
 * Determine whether the given `value` is an object.
 *
 * @param {*} value
 *
 * @returns {value is object}
 */
function isObject (value) {  
  return Object.prototype.toString.call(value) === '[object Object]'
}

You can now use the isObject method like this:

isObject({})  
isObject({ name: 'Future Studio' })  
// true

isObject()  
isObject(null)  
isObject([1, 2])  
isObject(new Date())  
// false

That’s it!

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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