Warn when cacheKey is unset and function.length > 1?

[fregante]

By default, only the memoized function's first argument is considered via strict equality comparison

I think mem should call console.warn in this case:

mem(async (host, path) => fetch(host + path))

Correct usage:

mem(async (host, path) => fetch(host + path), {
  cacheKey: args => args.join()
})

mem(async (host, path) => fetch(host + path), {
  cacheKey: JSON.stringify
})

[sindresorhus]

What if the parameter is there but does not change, so does not affect memoization? I don't think we can reliably detect incorrect usage here. And console.warn is not really something you want to be using in a reusable package.

[fregante]

What if the parameter is there but does not change

I think you're talking about, say, a function that accepts a (string, opts), where opts is always omitted by the caller, thus "never changes".

What the caller does with the function is irrelevant. Since memoization wraps the function, it should follow the function’s logic, not the caller’s. e.g.

export const normal = fetch;
export const memoized = mem(fetch);

If a function accepts a parameter, why would it "not change"? If that exists, it sounds like an exception.

And console.warn is not really something you want to be using in a reusable package

Agreed, but it sounds like a good warning and it's easy to silence:

console.warn(`
Function accepts 2 parameters, but only one is considered during mem.
Use \`cacheKey: ([x]) => x\` to silence this message or refer to the docs:
https://github.com/sindresorhus/mem#caching-strategy
`)

  mem(async (host, path) => fetch(host + path), {
+    cacheKey: ([x]) => x
  })

[sindresorhus]

If a function accepts a parameter, why would it "not change"? If that exists, it sounds like an exception.

For example, an API with a parameter where you always pass in the same value as you only use it that way. Yes, the correct way would be to set a custom cache key, but since the parameter is practically static, it feels moot.

[sindresorhus]

I'm ok with console.warn with a good message like the above.

[fregante]

Side quest: detecting functions with optional parameters/rest.

(url, opts = {}) => {} would not be detected by .length. It would be interesting to detect the real length. For rest parameters, the mere existence should trigger the warning;

await memQuantumSum(12, 45, 71)

I think however this requires a whole JS parser though 😰 so, no.

[fregante]

Wait a minute. TypeScript to the rescue? This is right up its alley and better than a runtime warning.