conditional-expression

v1.1.2

Published

2 years ago

JavaScript functional conditional expression

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ragnarecek

functional conditional expression match pattern matching

conditional-expression

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Providing 'match' function as a JavaScript functional conditional expression to replace conditional statements 'if' and 'switch' as an alternative to ternary operator expression.

GitHub repository: https://github.com/MartinGentleman/conditional-expression

Medium article explaining motivation and use for the package: How to replace switch and ternaries in functional JavaScript.

Install

npm install conditional-expression --save

Without ES6:

var match = require('conditional-expression').default;

match(1)
  .equals(1).then(function () {
    console.log('hello world');
  }).else(false);

With ES6:

import match from 'conditional-expression';

match(1)
  .equals(1).then(() => console.log('hello world'))
  .else(false);

Usage

First you call match function on an expression that you want to match:

match('functional  programming');

Next you choose how you want to match the expression by choosing appropriate matching function. For example:

match('functional programming')
    .includes('programming');

You follow up by calling 'then' function to tell it what to do in case of a positive match. You can pass a simple value as well as a function that will be automatically evaluated:

match('something')
    .with(/[a-z]/).then('awesome');

// or

match('functional programming')
    .includes('programming').then(() => console.log('awesome'));

You have the option of chaining:

match(42)
    .typeOf('string').then('it is a string')
    .equals('42').then('Answer to the Ultimate Question of Life, the Universe, and Everything');

And you finish everything by calling 'else' which is mandatory:

match('http://domain.tld')
    .on(url => url.substr(0, 5) === 'https').then('It is HTTPS indeed')
    .else('It is not HTTPS and that makes me sad :(');
// returns 'It is not HTTPS and that makes me sad :('

Once a match is found, no other matching is performed:

match('funny joke')
    .equals('sad').then('I am false')
    .with(/[a-z]/).then('I am true and I am the result')
    .includes('joke').then('I am true but I am not evaluated')
    .else('I just execute everything');
// returns 'I am true and I am the result'

Matching functions

on({function})

Evaluates as true if passed function returns boolean true, every other result of a function evaluates as false. Given function is also passed the expression over which we are matching as a parameter.

match({ grumpy: 'cat' })
    .on(x => x.grumpy === 'cat').then(true)
    .else(false);
// returns true

Internally this function is used to implement or other matching functions.

with({RegEx})

Evaluates as true based on passed regular expression.

match(73)
    .with(/[0-9]/).then(true)
    .else(false);
// returns true

equals({*})

Evaluates as true based on strict equality ===.

match('tortoise')
    .equals('tortoise').then(true)
    .else(false);
// returns true

includes({string})

Evaluates as true based on whether a substring is included. Always evaluates as false if not used to match on a string.

match('Martian')
    .includes('arti').then(true)
    .else(false);
// returns true

typeOf({string})

Evaluates as true based a type.

match({})
    .typeOf('object').then(true)
    .else(false);
// returns true

greaterThan({}), lessThan({}), atLeast({}), atMost({})

Evaluates as true based on sizes.

match(2).greaterThan(1).then(true).else(false);
// returns true

match('ab').lessThan('abc').then(true).else(false);
// returns true

match(2).atLeast(2).then(true).else(false);
// returns true

match(2).atLeast(3).then(true).else(false);
// returns true

match(2).atMost(2).then(true).else(false);
// returns true

match(2).atMost(1).then(true).else(false);
// returns true

thenMatch() nested matching

You can use nested matching to create subbranches for evaluation. Only 1 level deep nest is directly supported using thenMatch function.

const param = 'this is string';

match(param)
    .includes('this').thenMatch(param)
        .includes('string').then(true)
        .else(false);
    .else(false);
// returns true

Notice that thenMatch uses its own parameter and that else in the nested branch is still required.

To support deeper branching, you can pass match evaluation as a parameter to then function.

const param = 'this is string';

match(param)
    .includes('this').thenMatch(param)
        .includes('is').then(() => match(param)
            .includes('string').then(true)
            .else(false))
        .else(false);
    .else(false);
// returns true