About

An expression-oriented fluent alternative to javascript's if-statement that compiles away to ternary expressions

Example

Source:

processResult(
  If(2 === 2)
    .then('equals')
    .else('unequal')()
);

Compiled output:

processResult(2 === 2 ? 'equals' : 'unequal');

Why ?

1. Javascript if-else statements are not expressions.
2. Ternary expressions are ugly and even more so when nested.
3. Solutions like lodash.cond have unnecessary function invocation overhead and are less readable. To ensure lazy evaluation we need to wrap each branch in function.

This plugin is likely to become obsolete once do-expressions become supported by typescript (Relevant issue). If you don't care about type checking, then you can try out this babel-plugin.

Installation

This utility is implemented as a babel-macro.

Refer babel's setup instructions to learn how to setup your project to use babel for compilation.

1. Install babel-plugin-macros and if-expr.macro:

npm install --save-dev babel-plugin-macros if-expr.macro

2. Add babel-plugin-macros to .babelrc (if not already preset):

// .babelrc

module.exports = {
  presets: [
    // ... other presets
  ],
  plugins: [
    'babel-plugin-macros', // <-- REQUIRED
    // ... other plugins
  ],
};

3. Import if-expr.macro in your code:

// src/foo.js

import If from 'if-expr.macro';

const result = If(true).then(true)();

Features

• Branches are evaluated lazily

const result = If(true)
  .then(someFn())
  .else(someOtherFn())();

// result is what someFn returns
// someOtherFn is never called

• then/else branches are optional

const result = If(false).then(someFn())();

// someFn is never called
// result is undefined

• Multiple then/else branches are allowed:

const result = If(true)
  .then(someFn())
  .then(someOtherFn())();

// Both someFn and someOtherFn are called
// result is what someOtherFn returns

const result = If(false)
  .then(someFn())
  .elseIf(true)
  .then(someOtherFn())();

// Only someOtherFn is called
// result is what someOtherFn returns

• Side-effect only branches:

If(true)
  .thenDo(someFn(), someOtherFn(), yetAnotherFn())
  .thenDo(someOtherFn())();

// All of the functions are called (in specified order), but their return values are discareded
// The expression evaluates to undefined

• Side-effect only branches can be combined with then/else branches:

const result = If(true)
  .then(someFn())
  .thenDo(someOtherFn())();

// result is what someFn returns
// returned value (if any) of someOtherFn is discarded

Usage with TypeScript

This library is type-safe and comes with type definitions.

All code must be processed through babel. Compilation through tsc (only) is not supported.

Recommended babel configuration:

// .babelrc

module.exports = {
  presets: [
    '@babel/preset-typescript',
    // ... other presets
  ],
  plugins: [
    'babel-plugin-macros',
    // ... other plugins
  ],
};

Flow based type inference

One caveat is that TypeScript's flow-based type inference will not treat .then, .else branches same as normal if/else branches.

const a: undefined | string = getSomeValue();

if (a) {
  someFnThatExpectsString(a); // Not an error because TypeScript is smart enough to know
  // that a can not be undefined in this branch
}

const a: undefined | string = getSomeValue();

If(a).then(someFnThatexpectsString(a as string))()
                                     |________|
// We need to identify  -.             ^
// a as a string to      |------------/
// prevent type error   -'

AFAIK, currently there is no workaround for feasible.

Caveats

Every If/then/else chain fluent must end with a final function invocation without interruptions.

For example:

const a = 10;
const intermediate = If(a === 10).then('equal');
const result = intermediate();

Above code will fail to compile.

Because the entire If/then/else chain is compiled away, anything return by If/then/else can not be assigned, referenced, or used in any computation.

You may also like:

1. switch-expr.macro: Similar utility, providing a fluent expression-oriented macro replacement for switch statement

License

MIT