itch

Flexible and readable switch statement replacement library for JavaScript

Basic Usage

var itch = require('itch');

var price = itch(item)
   .match('Orange').then(0.59)
   .match('Apple').then(0.32)
   .match('Banana').then(0.48)
   .match('Cherry').then(3.00)
   .matchOneOf(['Mango', 'Papaya']).then(2.79)
   .scratch(999.99)

This code is functionally equivalent to the following alternative standard implementations:

Switch

var price = (function() {
   switch(item) {
      case 'Orange':
         return 0.59;
      case 'Apple':
         return 0.32;
      case 'Banana':
         return 0.48;
      case 'Cherry':
         return 3.00;
      case 'Mango':
      case 'Papaya':
         return 2.79;
      default:
         return 999.99;
   }
})();

If-else

var price = (function() {
   if (item === 'Orange') {
      return 0.59;
   } else if (item === 'Apple') {
      return 0.32;
   } else if (item === 'Banana') {
      return 0.48;
   } else if (item === 'Cherry') {
      return 3.00;
   } else if (item === 'Mango' || item === 'Papaya') {
      return 2.79;
   } else {
      return 999.99;
   }
})();

Object Literals

var price = {
   Orange: 0.59,
   Apple: 0.32,
   Banana: 0.48,
   Cherry: 3.00,
   Mango: 2.79,
   Papaya: 2.79
}[item] || 999.99;

Nested Ternaries

var price = (item === 'Orange')
   ? 0.59
   : (item === 'Apple')
      ? 0.32
      : (item === 'Banana')
         ? 0.48
         : (item === 'Cherry')
            ? 3.00
            : (item === 'Mango' || item === 'Papaya')
               ? 2.79
               : 999.99;

Syntax

itch(seed)
   .match(candidate).then(returnValue)
   .match(anotherCandidate).then(anotherReturnValue)
   // ...
   .scratch(defaultValue)

• An itch statement is a function chain that starts with a call to the itch function with a seed value.
• The itch() call is typically followed by a series of match().then() calls.
• match() accepts one argument, which is a candidate value to test for equality against the seed value.
• then() accepts one argument, which is the value that will be returned if the seed matches the current candidate.
• evaluate() may be used instead of then(). If the argument to evaluate() is a function, it will be evaluated only if the seed matches the current candidate.
• The chain must be concluded with a scratch() call, whose sole argument is the default return value, should none of the candidates match the seed.

Custom Matchers

Add a using() call in the chain before a match() to use a custom matching function thereafter:

itch(-1)
   .using(function(a, b) {
      return b === -a;
   })
   .match(1).then(true)
   .scratch(false);
// returns true

The following are equivalent:

itch(2)
   .using(function(a, b) {
      return b.some(function(bb) {
         return bb === a;
      });
   })
   .match([1, 2, 3]).then(true)
   .scratch(false);
// returns true

itch(2)
   .matchOneOf([1, 2, 3]).then(true)
   .scratch(false);
// returns true

Evaluate

Often you will want to use the return value of a function call rather than a simple value as an argument to a then() function. In these cases, you should wrap the function call in an anonymous function and pass it to execute() instead. If a function invocation is passed to a then() function it will execute even if the corresponding candidate does not match, whereas a function passed to then() will simply be returned without being evaluated. evaluate() allows itch to be used in a greater number of scenarios.

Similarly scratchEvaluate() can be used instead of scratch() in these cases.

scratchEvaluate() : scratch() :: evaluate() : then()

// ES2015
return itch(command)
   .match('log in').evaluate(() => http.post('login', credentials))
   .match('log out').evaluate(() => http.post('logout'))
   .scratchEvaluate(() => Promise.reject(new Error('Invalid command')));