Create a generic function for partial application from the right

Problem

You want to create a generic function, that produces for a given function the partial applied function, applying parameters from the right.

Ingredients

• 2 functions
• the apply method
• the slice method

Directions

1. Given: a generic function for partial application (see recipe 9) …

   function partial(fn /*, args...*/) {
     var args = Array.prototype.slice.call(arguments, 1);
     return function() {
       return fn.apply(this, args.concat(Array.prototype.slice.call(arguments, 0)));
     };
   }

2. … which could also be written like this:

   function partial(fn /*, args...*/) {
     var args = Array.prototype.slice.call(arguments, 1);
     return function() {
       var argsInner = Array.prototype.slice.call(arguments, 0);
       var argsAll = args.concat(argsInner);
       return fn.apply(this, argsAll);
     };
   }

3. To apply parameters from the right you only need to change the order how the argsInner and argsAll arrays are concatenated.

   function partial(fn /*, args...*/) {
     var args = Array.prototype.slice.call(arguments, 1);
     return function() {
       var argsInner = Array.prototype.slice.call(arguments, 0);
       var argsAll = argsInner.concat(args);
       return fn.apply(this, argsAll);
     };
   }

4. Optional: write this again as an one-liner:

   function partial(fn /*, args...*/) {
     var args = Array.prototype.slice.call(arguments, 1);
     return function() {
       return fn.apply(this, Array.prototype.slice.call(arguments, 0).concat(args));
     };
   }

5. Voilá, a perfect … oh, wait, you might want to rename the function to partialRight().

   function partialRight(fn /*, args...*/) {
     var args = Array.prototype.slice.call(arguments, 1);
     return function() {
       return fn.apply(this, Array.prototype.slice.call(arguments, 0).concat(args));
     };
   }

6. Voilá, a perfect generic function that creates partial applicable functions starting from the right.

   function createPerson(firstName, lastName) {
     return {
      firstName: firstName,
      lastName: lastName
     }
   }
   var createPersonWithLastNameDoe = partialRight(createPerson, 'Doe');
   var johnDoe = createPersonWithLastNameDoe('John');
   console.log(johnDoe.firstName);  // "John"
   console.log(johnDoe.lastName);   // "Doe"

Alternative recipes

• Since ES2015: use rest parameters as shown in recipe 15.

  const partial = (fn, ...args) => (...argsInner) => fn(...args, ...argsInner);
  const partialRight = (fn, ...args) => (...argsInner) => fn(...argsInner, ...args);