Partial Application

Introduction

Partial application pre-sets some arguments of a function, returning a new function that expects the remaining arguments. Currying makes partial application natural and powerful.

Basic Partial Application

const multiply = a => b => a * b;

// Partially apply `a = 2`
const double = multiply(2);
double(5);  // 10
double(10); // 20

// Partially apply `a = 3`
const triple = multiply(3);
triple(5);  // 15

Real-World Examples

Configuring API Clients

const apiRequest = baseUrl => endpoint => fetch(`${baseUrl}${endpoint}`).then(r => r.json());

// Create a specialized GitHub API client
const github = apiRequest('https://api.github.com');
const gitlab = apiRequest('https://gitlab.com/api/v4');

github('/users/octocat');
gitlab('/projects');

Logging with Levels

const log = level => message => console.log(`[${level}] ${message}`);

const info = log('INFO');
const warn = log('WARN');
const error = log('ERROR');

info('Server started');  // [INFO] Server started
warn('Disk space low');  // [WARN] Disk space low
error('Crash!');         // [ERROR] Crash!

Event Formatting

const formatWith = locale => date => new Intl.DateTimeFormat(locale).format(date);

const formatUS = formatWith('en-US');
const formatDE = formatWith('de-DE');
const formatJP = formatWith('ja-JP');

formatUS(new Date('2024-01-15')); // 1/15/2024
formatDE(new Date('2024-01-15')); // 15.1.2024

Partial Application vs Currying

While related, they’re different:

Currying: transforms fn(a, b, c) → fn(a)(b)(c)
Partial Application: pre-fills arguments: partial(fn, a, b) → fn(c)

// Generic partial application (not currying-specific)
function partial(fn, ...presetArgs) {
  return function(...laterArgs) {
    return fn(...presetArgs, ...laterArgs);
  };
}

function greet(greeting, name) {
  return `${greeting}, ${name}!`;
}

const sayHello = partial(greet, 'Hello');
const sayGoodbye = partial(greet, 'Goodbye');

sayHello('Alice');  // 'Hello, Alice!'
sayGoodbye('Bob');  // 'Goodbye, Bob!'

With .bind()

JavaScript’s .bind() also enables partial application:

function greet(greeting, punctuation, name) {
  return `${greeting}, ${name}${punctuation}`;
}

const sayHello = greet.bind(null, 'Hello');
sayHello('!', 'Alice'); // 'Hello, Alice!'

const sayHelloExclamation = greet.bind(null, 'Hello', '!');
sayHelloExclamation('Alice'); // 'Hello, Alice!'

Summary

Partial application pre-sets arguments, returning a function for the rest
Currying makes partial application seamless
.bind() can also partially apply arguments
Powerful for creating specialized functions from general ones

Partial Application

Partial Application

Introduction

Basic Partial Application

Real-World Examples

Configuring API Clients

Logging with Levels

Event Formatting

Partial Application vs Currying

With .bind()

Summary

Further Reading