Consistent input validation, and a CHANGELOG

[tbeseda]

Follow-up to #1, and the last thing before 0.2.0.

The problem

treeify had three different reactions to the same class of mistake:

input	before	after
[]	''	''
[undefined]	''	throws
null, 'root', {}	''	throws
[1, ['child']]	throws Error	throws TypeError
['root', 42, 'b']	42 silently dropped	42 renders
['root', [42, 'b']]	42 silently dropped	42 renders

A number where a string belongs threw at index 0 and was silently swallowed everywhere else. Input that wasn't an array at all was the quietest outcome of the lot, even though it's the one most likely to be a real bug.

The policy

Throw for what can't be rendered, render everything else.

• Not an array → TypeError: array-treeify: expected an array, received null
• First element isn't a string → TypeError: array-treeify: expected the first element to be a string, received number (1)
• Anything that isn't an array is a label, stringified at any depth, and can be a parent node
• treeify([]) still returns '' — a useful "nothing to render"

No error class is exported. The TypeError message is the API.

treeify(['deploys', [2025, ['spring', 'summer'], 2026, ['q1']]])
// deploys
// ├─ 2025
// │  ├─ spring
// │  └─ summer
// └─ 2026
//    └─ q1

number, bigint and boolean are now accepted as labels without a cast. The exported TreeInput type is unchanged.

Docs

The README claimed TypeScript "cannot enforce at the type level that the first element is a string". That isn't true — [string, ...(string | TreeInput)[]] does exactly that. It would rule out building a tree with push, which is the real reason for the permissive type, so the docs now say that instead. Added sections on labels and errors; the new example is covered by the README test suite.

CHANGELOG

Added, in Keep a Changelog format, backfilled from the tags (0.1.1 through 0.1.5). The breaking changes from this PR and from #1 are collected under Unreleased.

Verified

• 23/23 tests pass; npm run check clean.
• Valid trees render exactly as before: 15,000 renders of randomly generated string trees across all three option shapes (default, plain, custom chars) are byte-identical to the previous implementation.