Check if Variable is of Function Type using JavaScript

Check if Variable is of Function Type #

To check if a variable is of type function, use the typeof operator, e.g. typeof myVariable === 'function'. The typeof operator returns a string that indicates the type of the value. If the type of the variable is a function, a string containing the word function is returned.

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function sum(a, b) {
  return a + b;
}

if (typeof sum === 'function') {
  console.log('✅ type is function');
} else {
  console.log('⛔️ type is NOT function');
}

We used the typeof operator to check if a variable has a type of function.

The typeof operator returns a string that indicates the type of value to the right hand side. Here are some examples.

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console.log(typeof (() => {})); // 👉️ "function"
console.log(typeof function () {}); // 👉️ "function"
console.log(typeof null); // 👉️ "object"
console.log(typeof []); // 👉️ "object"
console.log(typeof {}); // 👉️ "object"
console.log(typeof ''); // 👉️ "string"
console.log(typeof 0); // 👉️ "number"

For both arrow functions and named functions, the typeof operator returns a string containing the word function.

The typeof operator works in the same way when used with async functions.

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console.log(typeof (async () => {})); // 👉️ "function"
console.log(typeof async function () {}); // 👉️ "function"

Even if you use it with a variable that's not declared, you wouldn't get an error.

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if (typeof sum === 'function') {
  console.log('✅ type is function');
} else {
  console.log('⛔️ type is NOT function');
}

In this example, we checked for the type of the sum variable, which is not declared.

The typeof operator returned "undefined" and didn't throw an error.

Further Reading #

• How to Check if a Function is Defined in JavaScript
• Check if a Map has an Object Key using JavaScript