let-in.js - Let expressions introduced into JavaScript

This is still in Beta, so use at your own risk, API might change and break.

Let expressions make everything simpler. Creates smaller and more resuable functions.

letIn

Receives functions and calls the them in order, each function receives the previous results by order.

import { letIn } from 'let-in'

const result = letIn(
  () => 1,
  (a) => 2 + a,
  (a, b) => 3 + a + b,
  (a, b, c) => a + b + c,
)

console.log(result); // 11

Wrap letIn in a function to create a new function with the Let In Expression structure

import { letIn } from 'let-in'

const fn = (x, y) => letIn(
  () => 1 + x,
  (a) => 2 + a,
  (a, b) => 3 + a + b,
  (a, b, c) => a + b + c + y,
)

console.log(fn(1, 1)); // 16

Examples

Before

  const before = (a, b, c) => {
    const x1 = a ^ 2;
    const x2 = b / 3;
    const x3 = c - 1;
    return x1 + x2 + x3;
  }

After

  const after = (a, b, c) => letIn(
    () => a ^ 2,
    () => b / 3,
    () => c - 1,
    (x1, x2, x3) => x1 + x2 + x3
  )

After2

  const after = (a, b, c) => letIn(
    () => a ^ 2,
    () => b / 3,
    () => c - 1,
    (...xs) => xs.reduce((sum, x) => sum + x, 0);
  )

After3 + lambda-light

  const after = (a, b, c) => letIn(
    () => a ^ 2,
    () => b / 3,
    () => c - 1,
    (...xs) => reduce(sum, 0, xs);
  )