JavaScript - Higher-Order Functions - First Class Functions

The main difference between a mathematical function[or pure functional language function] and the notion of a "function" used in imperative programming[widely spread industrily used programming languages] is that imperative functions [or methods] can have side effects, changing the value of already calculated computations, i.e. the same language expression can result in different values at different times depending on the state of the executing program.

Conversely, in functional code, the output value of a function depends only on the arguments that are input to the function, so calling a function f twice with the same value for an argument x will produce the same result f(x) both times.

Functions are higher-order when they can take other functions as arguments, and return them as results. (Operators in mathematics, such as the differential operator d / dx that produces the derivative in calculus when applied to a function f, are examples of this.)

Higher-order functions enable currying, a technique in which a function is applied to its arguments one at a time, with each application returning a new function that accepts the next argument.

Higher order functions in Java script.

sayHello = function(f1){

alert(f1('Power Of JS')('Easy Higher Order Functions!'));

};

sayHello(function(prefix){

getPrefix = function(){

return prefix;

};

return function(a){

return getPrefix()+' - ' +a;

}; } );