Function premap

Passes inputs through a function before handing them to the given index. If the function returns undefined, the input is ignored.

Example

const collection = new Collection<Readonly<{
  name: string,
  age: number,
}>>();

// Ensure all names are unique
const ixUniqueNames = collection.registerIndex(
  premap(
    p.name,
    uniqueHashIndex()
  )
);

// Maintain the number of adults
const ixAdultCount = collection.registerIndex(
  premap(
    p => p.age >= 18 ? p : undefined,
    countIndex()
  )
);

console.log(ixAdultCount.get().value);
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