derive state from selectors (filters)

create-selector.js

// 4 June 2019
// ES5 version of create and test taken from Nick Scialli's articl,
// "Calculating Derived State in JavaScript Using Selectors"
// @see https://nick.scialli.me/calculating-derived-state-in-javascript-using-selectors/

function createSelector(functions) {
  var len = arguments.length;
  if (!len || (len == 1 && typeof functions == 'function')) {
    throw new SyntaxError("expects more than one function: " + functions);
  }
  
  var funcs = len > 1 ? [].slice.call(arguments) : [].slice.call(functions)
  
  var last = funcs.length > 1 ? funcs.pop() : null;
  
  return function(state) {
    var inputs = funcs.map(function(func) {      
      return func(state);
    });
    
    return last ? last.apply(0, inputs) : inputs;
  };
};


/* test it out */


function allowed(who) { return who.age >= 21 || who.employee; }

function friendly(who) { return who.isFriendly; }

function both(allowed, friendly) { return allowed && friendly; }

var testVarArgs = createSelector(allowed, friendly, both);
var testVarArray = createSelector([ allowed, friendly, both ]);

var tests = [
  {
    name: "TTT",
    age: 22,
    employee: true,
    isFriendly: true
  },
  {
    name: "TTF",
    age: 22,
    employee: true,
    isFriendly: false
  },
  {
    name: "TFT",
    age: 22,
    employee: false,
    isFriendly: true
  },
  {
    name: "TFF",
    age: 22,
    employee: false,
    isFriendly: false
  },
  {
    name: "FTT",
    age: 19,
    employee: true,
    isFriendly: true
  },
  {
    name: "FFT",
    age: 19,
    employee: false,
    isFriendly: true
  },
  {
    name: "FTF",
    age: 19,
    employee: true,
    isFriendly: false
  },
  {
    name: "FFF",
    age: 19,
    employee: false,
    isFriendly: false
  }
];


console.log(
  tests.map(test => testVarArgs(test))
);

console.log(
  tests.map(test => testVarArray(test))
);

~(function enforcesTwoFunctions() {
  var testVarOne;
  var error = 'No errors. All is fine.'
  try { testVarOne = createSelector(allowed); }
  catch (e) { 
    error = e
  }
  finally {
    console.error(error);
  }
}());

/*
// Should see:
// [ true, false, true, false, true, false, false, false ]
// [ true, false, true, false, true, false, false, false ]
// SyntaxError: "expects more than one function as argument: function allowed(who) { return who.age >= 21 || who.employee; }"
*/