Javascript null instanceof

But JavaScript functions can do more: they can even construct objects! To make a function construct objects, just use new keyword before the regular function invocation:. How can you check in JavaScript that a certain instance was created with a certain constructor? Welcome instanceof operator:. Starting ES, a better way to construct objects is by using the class syntax. For example, let's define a class Pet and then created an instance of it myPet :. As for more practical examples, you may find instanceof useful to determine the built-in special instances like regular expressions, arrays:.

In simple words, object instanceof Constructor evaluates to true if object is an instance of Constructor , but also if Constructor is the parent class of instance's class.

JavaScript is a loosely-typed language, meaning that there is no restriction on what type a variable can have. Quiz: What is the built-in constructor for which instanceof for any object returns true? Start discussion. Specifically, in Item 8, he notes that in equals methods, one instanceof operator serves two purposes - it verifies that the argument is both non-null and of the correct type. BenThurley - The behavior is guaranteed by Java specifications past and present. I think Luke's point addresses the limitations of experimentation in determining the guaranteed behavior of the present.

Show 5 more comments. Matthew Read 1 1 gold badge 27 27 silver badges 46 46 bronze badges. Bozho Bozho k gold badges silver badges bronze badges.

Very good question indeed. I just tried for myself. Type Comparison Operator instanceof At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Jin Kwon Jin Kwon Kind of confusing. So what the hell is s? KaiWang s is just an object reference variable. It may refer an actually existing object "" or it may refer a the null literal reference.

I'm still confused. It can not be pointed to, like, an Integer. So it is still kind of a String, even it is a null. Just doesn't make much sense KaiWang You are confusing the variable type with the type of the actual object. Variables aren't instances; they're effectively just pointers.

KaiWang you have to imagine that in the call s instanceof String the s gets replaced with the actual value, so that would become "" instanceof String and null instanceof String. Thinking about it like this may make more sense. Show 2 more comments. L Joey 2 2 silver badges 7 7 bronze badges. RoflcoptrException RoflcoptrException Just as a tidbit : Even A null instanceof A will return false. Attila Tanyi Attila Tanyi 4, 4 4 gold badges 25 25 silver badges 31 31 bronze badges.

Normally, instanceof examines the prototype chain for the check. We can also set a custom logic in the static method Symbol. Most classes do not have Symbol. In that case, the standard logic is used: obj instanceOf Class checks whether Class.

In the example above rabbit. So the test of obj instanceof Class can be rephrased as Class. Only the chain of prototypes and Class. That can lead to interesting consequences when a prototype property is changed after the object is created. We can use it as an extended typeof and an alternative for instanceof. By specification , the built-in toString can be extracted from the object and executed in the context of any other value.

And its result depends on that value. Internally, the toString algorithm examines this and returns the corresponding result. More examples:. The behavior of Object toString can be customized using a special object property Symbol.