Add support for cache: 'while-pending' option

[Copilot]

Fixes #53

This PR adds support for cache: 'while-pending' as a more readable alternative to cache: false for disabling result caching while still deduplicating concurrent in-flight requests.

Problem

Previously, to enable in-flight deduplication without persistent caching, users had to write:

pMemoize(fn, {cache: false /* only while pending */})

The cache: false syntax is not self-documenting and requires comments to clarify its behavior.

Solution

This PR implements cache: 'while-pending' as discussed in the issue comments:

pMemoize(fn, {cache: 'while-pending'})

This is much more readable and self-explanatory - it clearly indicates that results are only memoized while the promise is pending (in-flight), but not after resolution.

Implementation Details

• The Options type now accepts cache: 'while-pending' in addition to false and cache storage objects
• Internally, 'while-pending' is normalized to false for processing, maintaining all existing behavior
• Both cache: false and cache: 'while-pending' behave identically - they disable persistent caching but still deduplicate concurrent requests
• The change is fully backward compatible - existing code using cache: false continues to work unchanged

Changes

• Updated TypeScript types to accept 'while-pending' as a cache option
• Added normalization logic to treat 'while-pending' the same as false
• Added comprehensive tests for the new option
• Updated documentation in README

All tests pass (27/27) ✅

Original prompt

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This section details on the original issue you should resolve

<issue_title>Export "in-flight only memoizer"</issue_title>
<issue_description>- Follows #20 (comment)

I don't know what this means:

pMemoize(fn, {cache: false})

I have to document it every time:

pMemoize(fn, {cache: false /* only while pending */})

The module can export a more readable helper instead:

export const pMemoizePending = (fn, opts) => pMemoize(fn, {...opts, cache: false})

Note: the types of opts should exclude cache

Some tags could also be added to package.json</issue_description>

<agent_instructions>create cache: "while-pending"</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@sindresorhus I would prefer to fix main signature. Maybe a new option instead of setting `cache` to `false`? The problem with exporting a separate method is that we have to explain it doesn't have a `cache` setting and also need to add yet another method for a decorator version.

@Richienb Any thoughts on this?</comment_new>
<comment_new>@fregante
I don't think that's a big ask. The documentation would look like

memInFlight(fn, options)

Like mem, but just while pending. The options are the same except you can't pass a custom cache.

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Same for the decorator. I do agree it's not optimal but… in the code it still reads better than any option could, IMHO</comment_new>
<comment_new>@fregante
I think cache: 'while-pending' is clear enough. "Pending" alone doesn't give enough information and still requires a comment</comment_new>
<comment_new>@sindresorhus
Let's go with cache: 'while-pending'.</comment_new>

Fixes #53

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[fregante]

@sindresorhus since false and while-pending are equivalent, should we deprecate and then remove "false"?

[sindresorhus]

We can soft-deprecate it by removing it from the readme, but I don't see a strong urge to officially deprecate it for existing users yet.