safeCompose

Create robust function compositions worrilessly

Why safeCompose

To cover all possible paths and shapes data can take in a composition can be very energy and time consuming. With safeCompose we only have to think about our happy path.

Installation

npm install safe-compose

Import

Import the function thusly:

import safeCompose from 'safe-compose'

How safeCompose works

I think the best way to explain how safeCompose works is by going through a few examples.

In the example below we run our composition with undefined. The first function to receive our data is head, which expects an array. Calling head with undefined will lead to an exception, but do not fret – safeCompose uses try...catch for all functions in the composition. safeCompose has an output variable that will be assigned either the output value of the current function or undefined in case the function fails. In this case all our functions will fail, so the output value of the composition will be undefined.

const head = xs => xs[0]
const toUpper = x => x.toUpperCase()
const trim = x => x.trim()
 
safeCompose(
  trim, // output = undefined
  toUpper, // output = undefined
  head // output = undefined
)(undefined) // => undefined

To provide a fallback you can use a function that returns the fallback value if a condition isn't met.

safeCompose(
  x => typeof x === 'string' ? x : 'FALLBACK',
  trim, // output = undefined
  toUpper, // output = undefined
  head // output = undefined
)(undefined) // => 'FALLBACK'

You can also use a non-function value as a fallback. Note that only the first argument can be a non-function value.

safeCompose(
  'FALLBACK',
  trim, // output = undefined
  toUpper, // output = undefined
  head // output = undefined
)(undefined) // => 'FALLBACK'

To recover from a failure mid-composition you can do something like this:

safeCompose(
  trim, // output = 'FALLBACK'
  toUpper, // output = '  FALLBACK'
  x => x || '  fallback', // output = '  fallback'
  head // output = undefined
)(undefined) // => 'FALLBACK'

Performance

safeCompose is very fast since we don't need to check input values in the composition. If you'd like to see some numbers you can check out the Stackblitz page. To see the results you have to open your browsers console.