`map` applies a function to each value in a structure and returns a corresponding structure. With an array, it transforms elements without changing the original array. With a composable object or observable, the same idea transforms a contained value or an emitted event while preserving the kind of structure. section 4.1 section 5.4 section 9.4.3
From loops to a data pipeline
A wallet balance can be calculated by filtering out genesis blocks, extracting their transactions, mapping each transaction to its effect on one address, and reducing those effects to a total. `flatMap(prop('data'))` can replace a separate `map(prop('data')).flat()` when transaction arrays must be flattened. The pipeline makes the data explicit rather than repeatedly changing a balance inside nested loops. section 4.7
Mapping is a form of composition: each transformation supplies a value to the next. A curried `map` can be used inside `compose`, while an object’s `map` method provides a similar way to chain transformations. `flatMap` serves a different need: it chains functions that themselves return a structure, such as validation checks returning `Validation` results. section 4.7 section 5.3 section 5.5.5
Transforming a validated value
A `Success` value can be mapped through a calculation and remain a `Success`: `map.call(Success.of(2), x => x * 2)` yields `Success(4)`. A shared map contract also lets standalone library functions delegate to an object’s own `map` method. section 5.6 section 5.5.8
Listing 9.18 Custom map operator
const map = curry( #1
(fn, stream) =>
new Observable(observer => { #2
const subs = stream.subscribe({ #3
next(value) {
try {
observer.next(fn(value)); #4
}
catch (err) {
observer.error(err);
}
},
error(e) {
observer.error(e); #5
},
complete() {
observer.complete(); #6
}
});
return () => subs.unsubscribe(); #7
});
);
An observable map subscribes to its source and sends `fn(value)` downstream as each value arrives. It also forwards errors and completion and provides an unsubscribe path, so mapping remains part of a composable event pipeline. section 9.4.3
