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I think it might be only performance case - http://jsperf.com/comparing-underscore-js-isfunction-with-typeof-function/2
And seems that typeof is faster.. so my question is - which is more appropriate to use?
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Using Strict Equal (===) operator: In JavaScript, '===' Operator is used to check whether two entities are of equal values as well as of equal type provides a boolean result. In this example, we use the '===' operator. This operator, called the Strict Equal operator, checks if the operands are of the same type.
Use the typeof operator to check if a function is defined, e.g. typeof myFunction === 'function' . The typeof operator returns a string that indicates the type of a value. If the function is not defined, the typeof operator returns "undefined" and doesn't throw an error.
typeof is a JavaScript keyword that will return the type of a variable when you call it. You can use this to validate function parameters or check if variables are defined. There are other uses as well. The typeof operator is useful because it is an easy way to check the type of a variable in your code.
Determining whether a relation is a function on a graph is relatively easy by using the vertical line test. If a vertical line crosses the relation on the graph only once in all locations, the relation is a function. However, if a vertical line crosses the relation more than once, the relation is not a function.
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 ?
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