@metreeca/flow - v0.9.21
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    Function toMap

    Creates a sink collecting items or extracted values into a map under extracted keys.

    • Creates a sink collecting the items of the feed into a map under extracted keys.

      Keys and values are made immutable as they are collected, in source order. Keys must be unique: an item whose key was already collected fails the sink rather than silently overwriting the entry standing under it. Keys are compared with SameValueZero semantics, so NaN matches itself and -0 matches 0.

      The returned map is frozen, with set, delete and clear shadowed by own properties that throw: entries cannot be altered through the map, although mutating methods invoked directly on Map.prototype still reach the internal slots backing them.

      Warning

      Accumulates the whole feed in memory. For large or infinite feeds, this may exhaust memory or never complete.

      Warning

      Freezing clones structured keys, giving them a fresh identity: entries are not reachable through the original key reference, and the same mutable key extracted twice yields two distinct entries rather than a duplicate key error. Extract structured keys as immutable values to keep their identity stable.

      Type Parameters

      • V

        The type of items in the feed

      • K

        The type of map keys

      Parameters

      • key: (item: V) => Awaitable<K>

        The function extracting the key from each item

      Returns Sink<V, ReadonlyMap<K, V>>

      A sink resolving to the deeply immutable read-only map pairing each extracted key with the item it was extracted from

      If two items yield the same key

      await pipe(
        (items({ id: 1, name: "Alice" }, { id: 2, name: "Bob" }))
        (toMap(x => x.id))
      );  // Map(2) { 1 => { id: 1, name: "Alice" }, 2 => { id: 2, name: "Bob" } }

    • Creates a sink collecting extracted values into a map under extracted keys.

      Collects like toMap without a value argument, pairing each key with an extracted value rather than with the item itself.

      Type Parameters

      • V

        The type of items in the feed

      • K

        The type of map keys

      • T

        The type of map values

      Parameters

      • key: (item: V) => Awaitable<K>

        The function extracting the key from each item

      • value: (item: V) => Awaitable<T>

        The function transforming each item into a map value

      Returns Sink<V, ReadonlyMap<K, T>>

      A sink resolving to the deeply immutable read-only map pairing each extracted key with the value extracted alongside it

      If two items yield the same key

      await pipe(
        (items({ id: 1, name: "Alice" }, { id: 2, name: "Bob" }))
        (toMap(x => x.id, x => x.name))
      );  // Map(2) { 1 => "Alice", 2 => "Bob" }

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