cont-flow-expr.switch

What if you could write something like:

const value = switch (query) {
  case candidate1:
    optionA;
  case candidate2:
    optionB;
  default:
    optionC;
};

Enter, cont-flow-expr.switch:

import hctiws from "cont-flow-expr.switch";

const value = hctiws(query)
  .case(candidate1).then(optionA)
  .case(candidate2).then(optionB)
  .default(optionC);

API

switch-case*-default

A switch-case expression can be written as:

import hctiws from "cont-flow-expr.switch";

const value = hctiws(query)
  .case(candidate1).then(optionA)
  .case(candidate2).then(optionB)
  // .case(candidateN-1).then(optionN-1)
  .default(optionZ);

with arbitrarily many uses of .case().then().

Branches can also be evaluated lazily:

import hctiws from "cont-flow-expr.switch";

const value = hctiws(query)
  .case(candidate1).thenDo(() => optionA)
  .case(candidate2).thenDo(() => optionB)
  .defaultDo(() => optionC);

Normal and lazy branches can be mixed arbitrarily, for example:

import hctiws from "cont-flow-expr.switch";

const value = hctiws(query)
  .case(candidate1).thenDo(() => optionA)
  .case(candidate2).then(optionB)
  .default(optionC);

Fallthrough is supported but has to be done explicitly, for example:

import hctiws from "cont-flow-expr.switch";

const value = hctiws(query)
  .case(candidate1).thenDoFallthrough(() => fn())
  .case(candidate2).then(optionA)
  .default(optionB);

// is the same as

let value;
switch (query) {
  case candidate1:
    fn();
  case candidate2:
    value = optionA;
    break;
  default:
    value = optionB;
}

Cases can be evaluated lazily by using .caseLazy(), for example:

import hctiws from "cont-flow-expr.switch";

const value = hctiws(query)
  .case(candidate1).then(optionA)
  .caseLazy(() => candidate2).then(optionB) // not run if `query` matches `candidate1`
  .default(optionC);

TypeScript

TypeScript support is included and can be used to ensure all branches evaluate to the same type (union). Type parameters are required for this to work. For example:

import hctiws from "cont-flow-expr.switch";

const query = 3.14, candidate1 = 3, candidate2 = 14;
const optionA = "foo", optionB = "bar", optionC = "baz", optionD = 42;

// Works
const aString = hctiws<number, string>(query)
  .case(candidate1).then(optionA)
  .case(candidate2).then(optionB)
  .default(optionC);

// Fails
const fails = hctiws<number, string>(query)
  .case(candidate1).then(optionA)
  .case(candidate2).then(optionB)
  .default(optionD);
  //       ~~~~~~~> TypeError:
  //                Argument of type 'number' is not assignable to parameter of type 'string'.

// Works
const aStringOrNumber = hctiws<number, string | number>(query)
  .case(candidate1).then(optionA)
  .case(candidate2).then(optionB)
  .default(optionD);

Benchmarks

The benchmarks in this project - which are powered by Benchmark.js - can be used to compare this library against vanilla JavaScript as well as alternative libraries. Here's a sample result:

vanilla x 1,411,831,771 ops/sec ±0.15% (98 runs sampled)
cont-flow-expr x 15,601,299 ops/sec ±0.83% (94 runs sampled)

Related

• cont-flow-expr.if - If statements as expressions.
• cont-flow-expr.try - Try statements as expressions.
• cont-flow-expr - Combines this and all of the above into one convenient package.

License

The source code is licensed under the ISC license. The documentation text is licensed under CC BY 4.0.