JavaScript Program to Check If a Variable is of Function Type

Introduction

In JavaScript, functions are first-class citizens. They can be passed as arguments to other functions, returned as values from other functions, and assigned to variables. This flexibility necessitates the ability to distinguish whether a variable is a function to ensure proper handling in code logic.

This article will help you learn to check whether a variable is a function using various techniques in JavaScript. Explore practical code examples that demonstrate how to perform these checks effectively, ensuring robust program execution and avoiding errors.

Check If a Variable is a Function using typeof Operator

Basic Function Type Check

1. Define a variable with a function value.

2. Use the typeof operator to determine if the variable is a function.

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   function myFunction() {
       return "Hello, world!";
   }
   
   let checkFunction = typeof myFunction === 'function';
   console.log(checkFunction);
   

   This code checks if myFunction is of type 'function'. The typeof operator returns a string indicating the type of the operand. Here, it returns true because myFunction is indeed a function. This is the easiest method to check if a JavaScript variable is a function.

Using typeof with Non-Function Variable

1. Assign a non-function value to a variable.

2. Apply the typeof operator to check if the variable is a function.

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   let myVariable = 42;
   let isFunction = typeof myVariable === 'function';
   console.log(isFunction);
   

   Since myVariable contains a number, the typeof operation returns false, indicating that this JavaScript variable is not a function.

Advanced Usage with Higher-Order Functions

Passing a Function as an Argument

1. Create a higher-order function that checks if its argument is a function.

2. Pass various types of variables to see the results.

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   function checkIfFunction(input) {
       return typeof input === 'function';
   }
   
   console.log(checkIfFunction(function() {})); // true
   console.log(checkIfFunction(12345));         // false
   console.log(checkIfFunction('Hello'));      // false
   

   This snippet demonstrates checking the type of various inputs using a reusable method. Only the function type passes the test, helping us check if a JavaScript variable is a function.

Using Function.prototype.toString

Confirm Function Source Code

1. Identify the necessity to differentiate between native and user-defined functions.

2. Utilize Function.prototype.toString and search to achieve this.

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   function userFunction() {
       return true;
   }
   
   let isNativeFunction = Function.prototype.toString.call(userFunction)
                          .search(/\[native code\]/) !== -1;
   console.log(isNativeFunction); // false
   

   This code checks whether userFunction is a native JavaScript function. It's not, as indicated by the false result. Native functions contain [native code] within their string representation. It’s an advanced method to verify whether a variable is a native JavaScript function.

Conclusion

Understanding how to determine if a variable is a function in JavaScript is crucial for handling various operations, especially in dynamic and large-scale applications. By utilizing the typeof operator and the Function.prototype.toString method, you can robustly ascertain a variable's function status. Implement these methods to maintain code integrity and avoid runtime errors due to misinterpretation of data types. Through these examples, ensure your JavaScript code remains reliable, clear, and functional.