Why use typeof for identifying a function?

Question

Are there any significant reasons for using

typeof variable === \'function\'

versus

!!variable.call

for d

Answer 1

jQuery's isFunction avoids the RegExp problem you mention by toString-ing the object and checking the result against a map of known types. From the latest source, here's the map:

// Populate the class2type map
jQuery.each("Boolean Number String Function Array Date RegExp Object".split(" "), function(i, name) {
    class2type[ "[object " + name + "]" ] = name.toLowerCase();
});

And here's how it's used:

type: function( obj ) {
    return obj == null ?
        String( obj ) :
        class2type[ toString.call(obj) ] || "object";
},

// See test/unit/core.js for details concerning isFunction.
// Since version 1.3, DOM methods and functions like alert
// aren't supported. They return false on IE (#2968).
isFunction: function( obj ) {
    return jQuery.type(obj) === "function";
},

You can learn a lot reading the jQuery source.

• http://james.padolsey.com/jquery/#v=1.5.0&fn=jQuery.isFunction
• http://james.padolsey.com/jquery/#v=1.5.0&fn=jQuery.type