How to Check if a Variable is of Function Type

There are several methods in JavaScript that can check whether a variable is of function or not. Let’s discuss each of them and find the fastest solution to the question.

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The typeof Operator

You can use the typeof operator to check the variable. It returns a string which indicates the type of the unevaluated operand.

Javascript typeof method
console.log(typeof '' === 'string'); console.log(typeof function() {} === 'function'); console.log(typeof true === 'boolean'); console.log(typeof Symbol() === 'symbol'); console.log(typeof {key: 1} === 'object');

The instanceof Operator

Another method can be the instanceof operator which checks the type of an object at run time. It return a corresponding boolean value, for example, either true or false to indicate if the object is of a particular type or not:

Javascript instanceof method
let str = new String(); let date = new Date(); console.log(str instanceof Object); console.log(str instanceof Date); console.log(str instanceof String); console.log(str instanceof Number); console.log(date instanceof Date); console.log(date instanceof Object); console.log(date instanceof String); console.log(date instanceof Number);
The typeof method appears to be the fastest in Chrome, but in Firefox the instanceof method is the winner.

The toString() Method

Another useful method is toString(). Each object has a toString() method, which is called when a value of string type is expected. If the method is not overridden, it will return the object type.

Javascript toString method
let arr = new Array("Html", "Css", "Javascript", "Git"); let str = arr.toString(); console.log("Showing string is : " + str );

JavaScript Functions

JavaScript functions are the main blocks of code that are designed to perform a particular task. Functions allow the code to be called many times without repetition. Variables that are defined inside a function cannot be accessed from anywhere outside that function, as it is defined only in the scope of the function.