pre and post conditions in js

pre-post-conditions.js

// this: https://twitter.com/raganwald/status/445938910808391680


// Wrap a function f (which accepts two arguments, and b) to enforce
// preconditions (if I understand them correctly) are guards on 
// parameter values

// You can also optionally add a postcondition afterwards
// though it'd be nicer to add this in the main preCondition function really

// conditionA and conditionB are boolean functions 
// f is our actual function

preCondition = function (f, conditionA, conditionB) {
    
    var conditionedF = function (a, b) {

        var returnValue;
    
        if (conditionA(a) && conditionB(b)) {
            
            returnValue = f(a,b);

            if (conditionedF.postCondition) {
                if (conditionedF.postCondition(returnValue)) {
                    return returnValue;
                }
                throw "WRONG AGAIN!";
            }

            return returnValue;
        }
        throw "WRONG!!!";
    
    }

    return conditionedF;

}


// And this is an example condition function
IntRange = function (low, high) {
    return function (val) {
        return (val > low && val < high);
    }
};



// so in use:
var myLimitedAddingFunction = preCondition( function (a, b) {return a+b;}, IntRange(0, 10), IntRange(-5, 0));

myLimitedAddingFunction(0, 0);
// > WRONG!!!
myLimitedAddingFunction(2, -3);
// > -1

// and optionally:
myLimitedAddingFunction.postCondition = IntRange(-4, -2);

myLimitedAddingFunction(2, -3);
// > WRONG AGAIN!