Install with

npm install passprint

Import with

const { pp } = require('passprint')

or

import { pp } from 'passprint'

Then use can put pp(expression) around any expression in your code, and it will return the value of expression unchanged, with the side-affect of logging it.

Imagine you have a bug in some code that looks like this and you want to inspect the value of a + x returned by the reducer.

const dotProd = (v1, v2) =>
  zip(v1, v2).map(pair => pair[0] * pair[1]).reduce((a, x) => a + x, 0)

Ideally you would use a debugger, but sometimes that not convenient and you want to just write out the value to the console, but to do that you normally would have to refactor the reducer code.

The pp function provides a convenient alternative, like so:

const { pp } = require('passprint')
...
const dotProdPP = (v1, v2) =>
  zip(v1, v2).map(pair => pair[0] * pair[1]).reduce((a, x) => pp(a + x), 0)

This will log out lines that look something like:

|||||||||MyClass.myFunction myFile.js:46:63 22ms 78878

where 22ms is the time elapsed since logging started and 78878 is the value of a + x. The number of | shows the depth of the call stack.

The pp simply returns the value passed to it, so it can be used with minimal perturbation to your code.

The default pp uses console.warn as the logging function.

If you want to use a different logger you can use the PP (uppercase) function to specify a different logger. For example to use console.log instead, you could do:

const { PP } = require('passprint')
const pp = PP(console.log)
...

Or you could specify a custom logger:

const { PP } = require('passprint')

const log = []
const pp = PP((message, value) => {
  log.push({ message, value })
})
...