How do you check if a variable is a function in JavaScript?

In JavaScript, there are several ways to check whether a variable is a function.

Here are three common ways:

Using the `typeof` operator:

function myFunc() {
  console.log("Hello World");
}

typeof myFunc === "function"; // true

The typeof operator returns a string indicating the type of the operand. When applied to a function, typeof returns the string “function”.

Using the `instanceof` operator:

function myFunc() {
  console.log("Hello World");
}

myFunc instanceof Function; // true

The instanceof operator checks whether an object belongs to a specific class. In this case, we’re checking whether myFunc belongs to the Function class.

Using `Object.prototype.toString.call()`:

function myFunc() {
  console.log("Hello World");
}

Object.prototype.toString.call(myFunc) === "[object Function]"; // true

Object.prototype.toString.call() returns a string representation of the object’s type. When called on a function, it returns the string “[object Function]“.

All of these methods should return true if the variable is a function. It’s worth noting that none of these methods will return true for function expressions that have not been assigned to a variable.