cont-flow-expr.if

What if you could write something like:

const value = if (condition) {
  optionA;
} else {
  optionB;
};

Enter, cont-flow-expr.if:

import fi from "cont-flow-expr.if";

const value = fi(condition)
  .then(optionA)
  .else(optionB);

API

if-else

A basic if-else expression can be written as:

import fi from "cont-flow-expr.if";

const value = fi(condition)
  .then(optionA)
  .else(optionB);

Branches can also be evaluated lazily:

import fi from "cont-flow-expr.if";

const value = fi(condition)
  .thenDo(() => optionA)
  .elseDo(() => optionB);

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

import fi from "cont-flow-expr.if";

const value = fi(condition)
  .thenDo(() => expensiveOption)
  .else(cheapOption);

if-elseif*-else

An if-elseif-else expression can be written as:

import fi from "cont-flow-expr.if";

const value = fi(condition1)
  .then(optionA)
  .elseIf(condition2)
  .then(optionB)
  // .elseIf(conditionN-1)
  // .then(optionN-1)
  .else(optionZ);

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

Here too branches can be evaluated lazily and mixed with normal branches arbitrarily:

import fi from "cont-flow-expr.if";

const value = fi(condition1)
  .then(optionA)
  .elseIf(condition2)
  .thenDo(() => optionB)
  .else(optionC);

Else conditions can be evaluated lazily by using .elseIfLazy(), for example:

import fi from "cont-flow-expr.if";

const value = fi(condition1)
  .then(optionA)
  .elseIfLazy(() => condition2) // not run if `condition1` holds
  .thenDo(() => optionB)
  .else(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 fi from "cont-flow-expr.if";

const condition = true, optionA = "foo", optionB = "bar", optionC = 42;

// Works
const aString = fi<string>(condition)
  .then(optionA)
  .else(optionB);

// Fails
const fails = fi<string>(condition)
  .then(optionA)
  .else(optionC);
  //    ~~~~~~~> TypeError:
  //             Argument of type 'number' is not assignable to parameter of type 'string'.

// Works
const aStringOrNumber = fi<string | number>(condition)
  .then(optionA)
  .else(optionC);

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 1,409,198,725 ops/sec ±0.16% (97 runs sampled)
cont-flow-expr x 68,040,893 ops/sec ±0.53% (96 runs sampled)
dolla-if x 60,359,107 ops/sec ±0.36% (99 runs sampled)

Related

• cont-flow-expr.switch - Switch 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.