Check type of callback parameter

Problem

You want to be sure that the value passed as the callback parameter is really a function.

Ingredients

• the typeof operator
• the === operator
• optional: the toString() and the call method

Directions

1. Given: a function that accepts a callback function.

   function someFunction(callback) {
     callback();
   }

2. Check that callback is of type function.

   function someFunction(callback) {
     if(typeof callback === 'function') {
       callback();
     }
   }

3. Optional: otherwise throw an error.

   function someFunction(callback) {
     if(typeof callback === 'function') {
       callback();
     } else {
       // handle it, throw error
       throw new TypeError('The callback parameter is no function.');
     }
   }

4. Voilá, a delicious type check of the callback parameter.

   someFunction(() => {console.log(4711)});  // 4711
   someFunction({});                         // TypeError

Variants

• Variant 1: use Object.prototype.toString as well.

  function someFunction(callback) {
    if(Object.prototype.toString.call(callback) === '[object Function]'
      || typeof callback === 'function') {
      callback();
    } else {
      // handle it, throw error
      throw new TypeError('The callback parameter is no function.');
    }
  }

Alternative recipes

• Use a type checking library like typly (after installing it via npm install typly).

  'use strict';
  const typly = require('typly');
  function someFunction(callback) {
    // only check
    if(typly.isFunction(callback)) {
      callback();
    } else {
      // handle it, throw error
      throw new TypeError('The callback parameter is no function.');
    }
  }

  'use strict';
  const typly = require('typly');
  function someFunction(callback) {
    // check and throw error when necessary
    typly.assertFunction(callback));
    callback();
  }