test_runner: add inlineSnapshot

[pmarchini]

This is a draft implementation related to this issue: #59231.

I'm opening this PR mainly to collect some feedback regarding this feature and potential implementations, solutions, or ideas!

Note: The proposed solution is working, but it still needs refinement.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[niedzielski]

These docs rock!

[niedzielski]

In my opinion, the ideal DX is to always assume a missing second parameter "wants to be updated" whether or not the --test-update-snapshots flag is passed. You can then get a really nice micro workflow on a given test without ever needing to pass the flag just by repeatedly deleting the param (in my IDE, I often delete the parameter just by hitting undo: ctrl-z, save, test / update, ctrl-z, save, test / update, ... until I get the shape I want).

[niedzielski]

Great name! I think this aligns really well to the current TestContext.assert.snapshot() API. Also, I'm pleasantly surprised I don't need to await here.

[niedzielski]

It would be really helpful if inlineSnapshot() supported wrapping. For example:

/**
 * Asserts bitmap is the expected hex value. The `expectedHex` arg is replaced
 * automatically when tests are run with `--test-update-snapshots`.
 */
function assertBitmapHex(ctx: TestContext, bmp: Uint8Array, expectedHex: string): string {
  const actualHex = [...bmp]
    .map(v => v.toString(16).padStart(2, '0'))
    .join('')
  ctx.assert.inlineSnapshot(actualHex, expectedHex) // <-- don't update me inline here!
}

test('', () => {
  // ...
  assertBitmapHex(bmp, '000000000000000000000000') // <-- I get updated inline here since I'm called at the top level test function.
})

It's possible to not use wrapping but then it gets pretty clumsy trying to fill out all the args in every test.

[JakobJingleheimer]

What if we merely augment the current t.assert.snapshot to accept a second argument?

t.assert.snapshot({ name: 'Alice', age: 30 }, `{ name: 'Alice', age: 30 }`)

And then to generate updates --test-update-snapshots=inline (where the default value is external or something).

[niedzielski]

accept a second argument

I believe TestContext.assert.snapshot() already accepts a second argument. Also, since you wouldn't pass a second argument initially, there wouldn't be a way to distinguish from a file call? I realize you're proposing a flag to account for that but a project may have many inline and external snapshot tests both so the distinction must be encoded at each call site.

[pmarchini]

What if we merely augment the current t.assert.snapshot to accept a second argument?

t.assert.snapshot({ name: 'Alice', age: 30 }, `{ name: 'Alice', age: 30 }`)

And then to generate updates --test-update-snapshots=inline (where the default value is external or something).

@JakobJingleheimer, IMHO I think we should decouple the two methods, as Jest does, for two reasons:

1. we can de‑risk what’s already in place
2. using a different method makes usage clearer and keeps it consistent with what’s already present in the ecosystem.