This is an important concept that we'll need later on—functions in JavaScript are actually data. This means that the following two ways to define a function are exactly the same:
function f(){return 1;}
var f = function(){return 1;}The second way of defining a function is known as function literal notation.
When you use the typeof operator on a variable that holds a function value, it returns the string "function".
>>> function f(){return 1;}
>>> typeof f"function"
So JavaScript functions are data, but a special kind of data with two important features:
They contain code
They are executable (can be invoked)
As you saw before, the way to execute a function is by adding parentheses after its name. As the next example demonstrates, this works regardless of how the function was defined. In the example, you can also see how a function is treated as a normal variable—it can be copied to a different variable and even deleted.
>>> var sum = function...