pass-when

pass-when

Functional and more robust switch statement replacement.

pass(8)
  .when(v => v > 10).to(v => `${v} is greater than 10`)
  .when(v => v < 10).to(v => `${v} is lower than 10`)
  .or().to(() => "is 10")
  .resolve();

// 8 is lower than 10

The idea behind it was to create simple way to solve problems of switch statement and turn it from statement to an expression. It works nicely when you're creating Redux reducer or writing custom validation with library like validator (what I'm doing when using redux-form).

Install

npm install --save pass-when

Usage

• resolve value with functor passed to to after first matched when
• resolve value with functor passed to to after or as default

import pass from "pass-when";

function compute(value) {
  return pass(value)
    .when(v => v > 10).to(v => `${v} is greater than 10`)
    .when(v => v < 10).to(v => `${v} is lower than 10`)
    .or().to(() => "is 10")
    .resolve();
}

compute(12); // 12 is greater than 10
compute(8);  // 8 is lower than 10
compute(10);  // is 10

• allows for additional conditions with andWhen and orWhen

function compute(value) {
  return pass(value)
    .when(v => v > 100)
    .andWhen(v => v % 2 === 0)
      .to(v => `${v} is greater than 100 and even`)
    .when(v => v > 100)
    .andWhen(v => v % 2 === 1)
      .to(v => `${v} is greater than 100 and odd`)
    .when(v => v < 50)
    .orWhen(v => v % 2 === 1)
      .to(v => `${v} is lower than 50 or odd`)
    .or()
      .to(v => `${v} is 50 - 100`)
    .resolve();
}

compute(201); // 201 is greater than 100 and odd
compute(200); // 200 is greater than 100 and even
compute(52);  // 52 is 50 - 100
compute(51);  // 51 is lower than 50 or odd
compute(10);  // 10 is lower than 50 or odd

• can resolve to promise

const response = { status: 404, data: { foo: "bar" } };
pass(response)
  .when(({ status }) => status >= 400 && status < 600)
    .to(res => ({ ...res, error: true }))
  .or()
    .to(res => ({ ...res, error: false }))
  .resolveToPromise()
  .then(res => {
    // { status: 404, data: { foo: "bar" }, error: true }
  });

• has negative operators whenNot, andWhenNot and orWhenNot corresponding to when, andWhen and orWhen

function compute(value) {
  return pass(value)
    .whenNot(v => v <= 100)
    .andWhenNot(v => v % 2 === 1)
      .to(v => `${v} is greater than 100 and even`)
    .whenNot(v => v <= 100)
    .andWhenNot(v => v % 2 === 0)
      .to(v => `${v} is greater than 100 and odd`)
    .whenNot(v => v >= 50)
    .orWhenNot(v => v % 2 === 0)
      .to(v => `${v} is lower than 50 or odd`)
    .or()
      .to(v => `${v} is 50 - 100`)
    .resolve();
}

compute(201); // 201 is greater than 100 and odd
compute(200); // 200 is greater than 100 and even
compute(52);  // 52 is 50 - 100
compute(51);  // 51 is lower than 50 or odd
compute(10);  // 10 is lower than 50 or odd

Todo

• Alternative syntax when(a, b)=>when(a).to(b) (issues/1)