const match = require('match-expression') const a = match('bar') .case('foo') .then(() => 'FOO') // You can chain .case clauses to associate multiple cases to the // same then callback. .case('bar') .case('baz') .then(() => 'BARBAZ') .default(() => 'DEFAULT') console.log(a) //------------------------------------------------------------ const maleNames = [ 'luke', 'carl', 'louis' ] let userName = 'carla' const genderResolution = match(userName, (name, test) => name.match(test)) // You can also pass multiple arguments to a single .case. In both cases, the // second argument passed to .then will be the value that caused the match. .case(/la$/) .then((name, matched) => `User ${name} was determined to be female (name matched ${matched}).`) .case(...maleNames) .case(/li$/) .then((name, matched) => `User ${name} was determined to be male (name matched ${matched}).`) .default((name) => `User ${name} is of unknown gender.`) console.log(genderResolution) //------------------------------------------------------------ // Sometimes you know that a value can only match a finite number of cases, // and you want to be explicit about what expressions are matched. In that case, // you can omit the .default clause by executing the function returned by .then. // Careful though, as if the value doesn't match anthing, the resolved value // will be `undefined` which can cause silent errors. To be safe, it can be a // good idea to include a .default clause which throws a TypeError. const nextTurn = match(51) .case(12, 21) .then(() => ({ page: 8 })) .case(51, 710) .then(() => ({ causeOfDeath: 'caught and used for experiments' })) .case(999) .then(() => ({ page: 42 }))() console.log(nextTurn.page ? `Go to page ${nextTurn.page}.` : `You are ${nextTurn.causeOfDeath}.`) //------------------------------------------------------------ // Since you can override the comparison function, cases don't need to be // "values". Here we define a new version of match that checks a value against // a series of predicates. const execMatch = a => match(a, (val, fn) => fn(val)) const pos = { x: -2, y: 5, z: 6 } const result = execMatch(pos) .case(({ x }) => x * 2) // The third argument to .then is the result of the comparison function's // application with the matched value, which allows us to reuse its result. .then((obj, _, res) => res) .case(({ y, z }) => y % 2 + z) .then((obj, _, res) => res) .case(({ x, y, z }) => x + y + z) .then(() => 0) .default((obj) => { throw new TypeError(`Malformed position: ${obj}`) }) console.log(result)