// This example is from the book _JavaScript: The Definitive Guide_.    
// Written by David Flanagan.  Copyright (c) 1996 O'Reilly & Associates.
// This example is provided WITHOUT WARRANTY either expressed or implied.
// You may study, use, modify, and distribute it for any purpose.        

// We define some simple functions here
function add(x,y) { return x + y; }
function subtract(x,y) { return x - y; }
function multiply(x,y) { return x * y; }
function divide(x,y) { return x / y; }

// Here's a function that takes one of the above functions
// as an argument and invokes it on two operands
function operate(operator, operand1, operand2) 
{
    return operator(operand1, operand2);
}

// We could invoke this function like this to compute
// the value (2+3) + (4*5):
var i =index.html operate(add, operate(add, 2, 3), operate(multiply, 4, 5));

// Now we store the functions defined above in an associative array
var operators = new Object();
operators["add"] = add;
operators["subtract"] = subtract;
operators["multiply"] = multiply;
operators["divide"] = divide;
operators["pow"] = Math.pow;  // works for predefined functions too.

// This function takes the name of an operator, looks up
// that operator in the array, and then invokes it on the
// supplied operands. Note the syntax used to invoke the
// operator function.
function operate2(op_name, operand1, operand2)
{
    if (operators[op_name] == null) return "unknown operator";
    else return operators[op_name](operand1, operand2);
}

// We could invoke this function as follows to compute
// the value ("hello" + " " + "world"):
var j = operate2("add", "hello", operate2("add", " ", "world"))

// Using the predefined Math.pow() function
var k = operate2("pow", 10, 2)