_.isFunction(a) vs. typeof a === 'function'? javascript
There is no reason not to use typeof.
Not only is it faster but the ECMAScript specification ensures that all functions have a type of "function" and that only functions can have a type of "function" :
This operator was specifically designed to get the type of a value, so why not use it ?
Firstly, Underscore doesn't use that implementation anymore. It optimizes to typeof unless typeof /./ returns function, as it did atleast in older versions of Chrome
You can find this in the source code: http://underscorejs.org/underscore.js
// Optimize `isFunction` if appropriate. if (typeof (/./) !== 'function') { _.isFunction = function(obj) { return typeof obj === 'function'; }; }New jsperf: http://jsperf.com/comparing-underscore-js-isfunction-with-typeof-function/3
It still shows quite a performance hit in FF (but MUCH less than the naive implementation you posted in the question), which is due to the overhead of a function call vs just inlining code.