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Daniel Joo
Daniel Joo

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The differences between typeof and instanceof in OO JS

I want to explain the differences between the typeof and instanceof operators in object-oriented JavaScript. Both of these operators can be used to find out more information about a certain piece of data.

typeof is an operator that can be used to verify what datatype a certain "operand" is. An "operand," as mentioned in the MDN documentation, is just a piece of data that is being manipulated or worked on. This operand can be pretty much anything, from a variable that contains a value to a function. When using typeof on an operand, all you need to is simply state typeof right before the operand.

Here are two examples:

const a = 6;
typeof a;

function hello(name) {
  return `hello, ${name}`

typeof hello
typeof hello()

In the first example, we are using typeof on the variable a, which contains the value of integer 6. Therefore, the return value is "number," because 6 is a number/integer.

The second example is a little more tricky. Here, we have the hello function. When using typeof on hello, we get the return value of function. This is because hello refers to the function body, or the code that is between the brackets, also known as the code block. If we call typeof on hello(), however, we get the value of string. This is because the function is now being invoked, and therefore returns the string within the code block.

The instanceof operator tests to see whether the operand is an instance, or object, of a certain class. In JavaScript, just like initialize method in Ruby, instances of a class can be created/initialized with certain attributes using the constructor method. This makes each instance of a class unique. Let's take a look at this example:

class Person {
   constructor(name,age) { = name
   this.age = age

const person = new Person('David',22)
const david = new Person



In this Person class, each new instance of Person will be created with a name and an age. this refers to the object/instance that is currently being created and initialized. this is similar in concept to self in Ruby.

With this knowledge of the constructor method in JavaScript, we can use the instanceof operator to verify whether the operand is actually an instance of a certain class. typeof returns a boolean value. Using the Person class above, we can use instanceof to verify whether person is an instance of the Person class.

console.log(person instanceof Person;)

console.log(david instanceof Person;)

When applying this to person and david, we can see that both of the expressions return the value of "true." This is because both are instances of the Person class.

To summarize, typeof and instanceof are neat little operators that can be utilized when trying to verify the properties/characteristics of objects and data in JavaScript.

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