INTER-2222: Upgrade to ESLint v10 flat config

[erayaydin]

Use @fingerprintjs/eslint-config-dx-team@3.0.0 which requires ESLint v10, exports a flat config array, and ships a type-aware config via its /type-checked export.

Dependency changes

Package	Before	After	Reason
eslint	8.57.0	^10.4.1	Required by dx-team config v3
@fingerprintjs/eslint-config-dx-team	^0.1.0	^3.0.0	ESLint upgrade, flat config, and type-aware rules (peer-declares eslint@^10 + typescript-eslint@^8)
eslint-config-next	14.1.3	^16.2.0	v14 uses @rushstack/eslint-patch which fails on ESLint v10; v16 is flat-config native
eslint-plugin-react-hooks	^4.6.0	^7.0.0	Required peer of eslint-config-next@16
eslint-plugin-prettier	^5.5.5	removed	Bundled in dx-team config
@eslint/eslintrc	-	^3.0.0	⚠️ Needed for FlatCompat in examples/preact. We can consider moving this to the example package.json file
typescript-eslint	-	^8.60.0	Explicit dep for parser override in root config

Config notes

settings.react.version: '18.2' instead of 'detect': the detect path in eslint-plugin-react@7.37.x calls context.getFilename() which was removed in ESLint v10.

Parser override for .js files: eslint-config-next@16 sets @babel/eslint-parser for all files, then restores @typescript-eslint/parser only for .ts/.tsx. The babel parser's eslint-scope@5.x scope manager lacks the addGlobals() method ESLint v10 requires, so we re-apply @typescript-eslint/parser for .js/.jsx files.

examples/preact — uses FlatCompat to wrap eslint-config-preact (no flat config support). plugin:@typescript-eslint/recommended is replaced with tseslint.configs.recommended (unified package v8) to support TypeScript 5.9

Type-aware linting (dx-team v3)

Switched the root config to the /type-checked export, which layers strictTypeChecked plus strict-boolean-expressions, prefer-nullish-coalescing, restrict-template-expressions, etc. on top of the base rules.

• Added tsconfig.eslint.json covering src, __tests__, and the root config files. The build tsconfig.json still only includes src, so tests/config would otherwise fail the type-aware parser with "not in project".
• The parserOptions.project block is ordered after nextConfig so its parser swap doesn't strip the typed parser off .tsx files; pure-JS config/scripts get disableTypeChecked.
• Most src fixes are mechanical (?? [], ??=, String(...) in templates, explicit nullish checks). Full pass verified: eslint --max-warnings 0, tsc --noEmit, 21/21 tests, and vite build all green.

Discussion point: test rule relaxation scoped to one rule

Tests only turn off @typescript-eslint/no-unsafe-assignment, because vitest's expect.any() matcher returns any. I measured what actually fires rather than copying the broader relaxation set from the node SDK — only this one triggers. Push back if you'd rather pre-emptively relax the full no-unsafe-* set for tests.

Discussion point: assertIsTruthy → assertIsDefined

strict-boolean-expressions flagged the !value check. Since the function asserts NonNullable<T> and throws "must not be null or undefined", I changed it to an explicit value === null || value === undefined and renamed it to match its real contract. Behaviour-adjacent but safe: both callers pass objects, never falsy primitives.

Discussion point: removed always-true if (env) guard

In fingerprint-provider.tsx env is typed non-nullable, so no-unnecessary-condition flagged the guard. Removed it and kept explicit empty/undefined handling for env.version. The react-sdk/.../react integration-info assertion exercises this path.

Code changes (ESLint v10 base rules)

@typescript-eslint/no-empty-object-type

Empty object type {} is now flagged; object is the correct replacement when you mean "no specific props".

no-promise-executor-return

__tests__/helpers.tsx

// before
new Promise(resolve => setTimeout(resolve, ms))
// after
new Promise(resolve => { setTimeout(resolve, ms) })

@typescript-eslint/no-non-null-assertion

DOM querySelector calls in tests suppressed with eslint-disable-next-line. The elements are asserted to exist by the test setup.

@typescript-eslint/no-explicit-any

• Test props typed as { env?: { name: string } } instead of any.
• render(...args: any[]) -> render(...args: unknown[]).

@typescript-eslint/ban-ts-comment

The rule now requires a description when suppressing comments.

@typescript-eslint/consistent-type-assertions

• const mock = fn as FunctionComponent -> const mock: FunctionComponent = fn
• Unavoidable widening/narrowing casts suppressed with eslint-disable-next-line.

no-useless-constructor

Empty constructor removed. It provided no behaviour and is redundant when the parent's constructor signature is identical.

@typescript-eslint/no-require-imports

CJS file using require() suppressed with a file-level disable. The script runs in Node
before any bundler, and cannot use ESM (depends on Node.js version).

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	82.68% (-0.53% 🔻)	105/127
🟡	Branches	63.77%	44/69
🟢	Functions	80% (-0.49% 🔻)	32/40
🟢	Lines	82.26% (-0.55% 🔻)	102/124

Show new covered files 🐣

St.❔	File	Statements	Branches	Functions	Lines
🔴	... / assert-is-defined.ts	50%	75%	100%	50%

Show files with reduced coverage 🔻

St.❔	File	Statements	Branches	Functions	Lines
🟢	... / fingerprint-provider.tsx	90.32% (-0.85% 🔻)	54.55% (-3.79% 🔻)	90%	90% (-0.91% 🔻)

Test suite run success

21 tests passing in 15 suites.

Report generated by 🧪jest coverage report action from d7ba0fb

Show full coverage report

St	File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
🟢	All files	82.67	63.76	80	82.25
🟢	src	88.7	75.86	83.33	88.33
🟢	detect-env.ts	90	83.33	100	90	15,87
🟢	env.ts	100	100	100	100
🟢	env.types.ts	85.71	40	50	83.33	14
🟡	...nt-context.ts	75	100	0	75	8
🟡	get-env.ts	60	0	100	60	8-9
🟢	ssr.ts	100	100	100	100
🟢	...sitor-data.ts	95.65	100	80	95.65	101
🟢	src/components	91.66	58.33	90.9	91.42
🟢	...-provider.tsx	90.32	54.54	90	90	54,59,92
🟢	...vironment.tsx	100	100	100	100
🟡	src/utils	58.62	50	63.63	58.62
🟡	...is-defined.ts	50	75	100	50	3
🟡	to-error.ts	66.66	50	100	66.66	6
🟢	use-const.ts	100	50	100	100	5
🟢	...mise-store.ts	92.3	75	100	92.3	7
🔴	wait-until.ts	0	0	0	0	8-19

[erayaydin]

Moving the PR to draft since we're planning to upgrade @fingerprintjs/eslint-config-dx-team to ^3.0.0 instead of ^2.0.0.

[github-actions]

⚠️ This PR doesn't contain any changesets. If there are user-facing changes, don't forget to run:

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