This document introduces partial application in JavaScript. It explains how functions can take other functions as parameters and return functions. This allows passing a fixed number of arguments to a function and returning a new function that takes the remaining arguments. Examples show partially applying a function that adds numbers by passing the fixed number to add to all calls. Underscore, Lodash, and other libraries provide utilities for partial application. Real-world uses and references for further information are provided.

1. Partial Application
In Javascript
(and why I like it)

2. Dude, who are you?
A’braham (Ah-brah-haum) Barakhyahu (Ba-rock-yahu)
Web developer (js, C# rest services)
jsGrunt
Vim kiddie
FP grasshopper

3. First Class Functions

4. Pass functions as parameters into
other functions
function shalom() {
return ‘Shalom’;
}
function sayToConsole(someGreeting) {
console.log(someGreeting());
}
sayToConsole(shalom);

5. Functions can return functions
function sayToConsole(greeting) {
return function(message) {
console.log( greeting + ‘ ‘ + message);
}
}
var greetBobAndSay = sayToConsole(‘Hi Bob!’);
greetBobAndSay(‘I have fantastic pants’);

6. Contrived Examples Ahead!

7. Instead of this
function add(x,y) {
return x + y;
}
for(var i = 0; i < 6 ; i++) {
console.log(add(5,i));
}

8. You could do this
function add(x) {
return function(y) {
return x + y;
}
}
var add5To = add(5);
for(var i = 0; i < 6 ; i++) {
console.log(add5To(i));

9. Or use underscore/lodash/
function add(x,y) {
return x + y;
}
var add5To = _.partial(add,5);
for(var i = 0; i < 6 ; i++) {
console.log(add5To(i));
}
[other functional library here]

10. little closer to a real-world example
sample

11. Refernces for more
Functional Javascript by Michael Fogus -
http://store.
kobobooks.com/en-us/books/Functional-JavaScript/
Lodash’s partialRight Underscore’s partial Currying vs. Partial Application -
http://lodash.com/docs#partialRight
http://underscorejs.org/#partial
http://www.2ality.com/2011/09/currying-
vs-part-eval.html
Partial Application in Javascript in-javascript/
http://ejohn.org/blog/partial-functions-

12. Thank you for you patience!
A’braham Barakhyahu
@blessYAHU
barakhyahu@gmail.com