How to Check for a Function in JavaScript
Want to know if a variable is a function in JavaScript? Simply use typeof — variables containing functions will always return "function".
New programmers to JavaScript are often not familiar with the idea that a function can be stored in a variable, just like any primitive type.
A function stored in a variable is often called a callback function, at least when that function is being passed into another function as a parameter.
“A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.” — MDN Docs
Callbacks are used in modern web APIs often with asynchronous promises.
Checking for a function
Checking for functions works as expected using the typeof keyword, which will return the string "function" for functions.
Using typeof will work for named functions, anonymous functions assigned to variables, and named functions assigned to variables.
Check out the code example:
Wrapping up
JavaScript functions can be invoked using the function or class keywords, or they can be created anonymously using arrow functions.
All JavaScript functions can be assigned to variables, and functions can be passed around to functions like any other JavaScript data type.
Passing functions as variables to other functions is very common in JavaScript, and the term callback function refers to this practice.
Specifically, the callback function is the function passed into another function as an argument, which is then used to complete some kind of action:
Conclusion & Further Reading
You can check for a function using the typeof keyword, which will return "function" if given the name of any JavaScript function.
If the function is assigned to a variable, check typeof the name of that variable, whether it was invoked using >let, var, or const.
If the function was named using the function or class keywords but not assigned to a variable, then check typeof the function’s given name.
Happy coding! 💻💯🏆💯🙌
Additional Resources
- The MDN Docs Guide to JavaScript Functions explains functions in-depth:
- GeeksforGeeks suggests checking for functions using the three methods of
instanceof, strict equality===, orobject.prototype.toString:
- GeeksforGeeks also covers checking to see if a function is declared:
- Chris Coyler keeps it even simpler when checking that a function exists:
if (typeof yourFunctionName == ‘function’) {yourFunctionName();}
Appendix: ES5 Specification
If you want to be precise, here is exactly how typeof works according to the ECMAScript (ES5) specification — note "object" and "function":
Dr. Derek Austin is the author of Career Programming: How You Can Become a Successful 6-Figure Programmer in 6 Months, now available on Amazon.
