".map" over console.log does not seem to show the stream

Summary

The following code does not seem to show the stream:

from([1,2]).map(x=>console.log(x))

Expected result

Expected: 1 followed by 2 in the console

Actual Result

In the Node REPL:

Stream {
  source: Map { f: [Function], source: ArraySource { array: [Object] } } }

Versions

node@6.9.2

• most.js: <version> most@1.3.0

Steps to reproduce

Node REPL

node
>most = require('most')
>most.from([1,2]).map(x=>console.log(x))
//=> Stream {
  source: Map { f: [Function], source: ArraySource { array: [Object] } } }

Code to reproduce

[Frikki]

@dmitriz As @TylorS explained, observe (f, stream) is really drain(tap(f, stream). f is your provided function that will perform a side effect on each item in the stream. Any return value of f is ignored. tab will then return a new stream with the same items as in the originally provided stream. The new stream is passed to drain, which returns a Promise. This Promise either fulfills when the stream ends without an error or is rejected if the stream ends with an error. Take note that the events of the stream are time-ordered.

Thus, if you have an “infinite” stream, e.g., a source that emits an event periodically, the provided function f will fire on every event, performing whatever side effect specified. Logically, the returned Promise will neither resolve nor reject (granted no errors occurred). After all, the stream is “infinite”. If the stream was finite (it ends), the code will then choose the path of either resolved or rejected Promise, if you so choose. As you see, the events are still consumed and passed to f.

You can easily map over the stream as in flyd, but you still need to consume the stream:

const stream = from([1, 2, 3])

const transformedStream = map(add2, stream)

function add2(value) { return value + 2 }

observe(console.log, transformedStream) // 3, 4, 5

Hope this helps (and that I didn’t provide any incorrect information in my explanation).

[briancavalier]

@dmitriz The original issue here seems to be resolved (observing the stream activates it), and it seems like @davidchase’s suggestion of looking at most-subject was helpful for your subsequent questions. So, I’m closing, but please re-open or create a new issue if there’s more we need to discuss.

Cheers!