The instance of operator tests whether the prototype property of a constructor appears anywhere in the prototype chain of an object. In a simpler languange, it tests if a variable is of a certain type. But it has a few caveats. Let us look at some examples.
Primitives
Strings and numbers are primitive values, not objects and therefore don't have a [[Prototype]], so it'll only work if you wrap them in regular objects.
Example
console.log(1 instanceof Number)
console.log(new Number(1) instanceof Number)
console.log("" instanceof String)
console.log(new String("") instanceof String)Output
false true false true
Constructible functions
Functions that return their objects(JS Classes) can have their objects checked using the instanceof operator.
Example
function Person(name) {
this.name = name
}
let john = new Person("John");
console.log(john instanceof Person)Output
true
Inheritence
JS supports prototypical inheritence, so if you check for instanceof for any class in the hierarchy, it'll return true.
Example
class Person {}
class Student extends Person {
constructor(name) {
super()
this.name = name
}
}
let john = new Student("John");
console.log(john instanceof Person)
console.log(john instanceof Student)Output
true true