Check if a variable is of function type

Sure underscore's way is more efficient, but the best way to check, when efficiency isn't an issue, is written on underscore's page linked by @Paul Rosania.

Inspired by underscore, the final isFunction function is as follows:

function isFunction(functionToCheck) {
 return functionToCheck && {}.toString.call(functionToCheck) === '[object Function]';
}

Note: This solution doesn't work for async functions, generators or proxied functions. Please see other answers for more up to date solutions.

if (typeof v === 'function') {
    // do something
}