It appears that the solution provided has obscured a larger issue and potentially a misunderstanding of how the map function functions in rxjs.
In traditional Javascript, the map() method directly manipulates an array by executing a function for each item within it. For example, [1,2,3].map(x => x + 1) results in a new array with transformed items.
Contrastingly, in rxjs, the map() operates differently from Array's map. Despite sharing the same name, the rxjs version works on single values within a stream rather than arrays.
RXJS deals with streams, which you already know! Suppose you have a stream of larvae (which could be either terrifying or adorable!). However, using http.get, you actually receive a stream with only one value - the complete response from your http call.
The original code snippet is annotated below:
public getButterfly(): Observable<Butterfly[]> {
return http.get<Larva[]>('url').pipe(
map( larva => {
// Don't confuse this 'map' with the standard javascript map
// Here, larva is likely an ARRAY returned from the HTTP call
// Attempting to access larva.dna will result in 'undefined'
// As dna is not a property of a javascript array
// Hence, returning a butterfly OBJECT with undefined DNA
return new Butterfly(larva.dna);
})
);
}
To address this issue, consider the following revised implementation:
public getButterflies() {
return http.get<Larva[]>('url').pipe(
map( larvae => larvae.map(larva => new Butterfly(larva.dna)))
);
}
This updated code snippet achieves the following:
- You receive an array (larvae) from the http call, specified as Larva[]
- The standard javascript
map function is applied to each item in the array to create a butterfly for each larva
- The new array of butterflies replaces the original array in the stream
- Note: The output type is inferred as
Butterfly[]
Providing <Larva, Butterfly> informs the compiler without altering the behavior. Additionally, specifying output types aids in identifying errors within the pipe.
Consider using
tap(x => console.log('Message', x)
within the pipe to log values at different stages:
public getButterflies() {
return http.get<Larva[]>('url').pipe(
tap( x => console.log('Before map', x) ),
map( larvae => larvae.map(larva => new Butterfly(larva.dna)),
tap( x => console.log('After map', x) )
));
}
Remember, "tap" allows you to peek inside the pipeline, just like a water tap lets you see what's flowing through a pipe!