@material-ui/popper.js/src/utils/isFunction.js

A fork of popper.js to avoid the deprecation warnings: https://github.com/mui-org/material-ui/issues/19358.

/**
 * Check if the given variable is a function
 * @method
 * @memberof Popper.Utils
 * @argument {Any} functionToCheck - variable to check
 * @returns {Boolean} answer to: is a function?
 */
export default function isFunction(functionToCheck) {
  const getType = {};
  return (
    functionToCheck &&
    getType.toString.call(functionToCheck) === '[object Function]'
  );
}