Registry / testing / eslint-plugin-expect-type

eslint-plugin-expect-type

library0.6.2jsnpmunverified

ESLint plugin that adds Twoslash-style type assertions ($ExpectType, $ExpectError, $ExpectTypeSnapshot) and the ^? hover type comment. Version 0.6.2 is the latest stable release as of early 2025, with active development and frequent releases (0.4.x to 0.6.x in rapid succession). Unlike runtime assertion libraries (expect-type, ts-expect), this plugin works at the ESLint linting level without test runners. Requires @typescript-eslint/parser ≥6, ESLint ≥7, and TypeScript ≥4. Unique for inline type assertions in comments, enabling type-level testing in any file linted by ESLint.

testing

npm install eslint-plugin-expect-type

github ↗package ↗

INSTALL

IMPORT

SIG · ESLINT-PLUGIN-EXPE

eslint-plugin-expect-type

testingjavascriptv0.6.2

Install

—

Import

—

Disk

—

Pass rate

0/ 6

Env Coverage0 / 6

glibc

18–22

musl

18–22

Install & Compatibility

Where this runs

tested against v? · npm install

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

node 18–226 runs

build_error

glibc

node 18–226 runs

build_error

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

default

✓ import expectType from 'eslint-plugin-expect-type/configs/recommended'

✗ const expectType = require('eslint-plugin-expect-type/configs/recommended')

CommonJS users must add .default: require('eslint-plugin-expect-type/configs/recommended').default

rules

✓ import { rules } from 'eslint-plugin-expect-type'

✗ const { rules } = require('eslint-plugin-expect-type')

rules is a named export, not available as a default import.

plugin

✓ import expectTypePlugin from 'eslint-plugin-expect-type'

✗ import { expectType } from 'eslint-plugin-expect-type'

The default export is the plugin object, not a named export named 'expectType'.

Shows how to configure the plugin in flat config and use $ExpectType, $ExpectError, ^?, and $ExpectTypeSnapshot.

// eslint.config.js import expectType from 'eslint-plugin-expect-type/configs/recommended'; export default [ expectType, { files: ['**/*.ts'], rules: { 'expect-type/expect': 'warn', 'expect-type/expect-error': 'error', }, }, ]; // my-file.ts let value = 9001; // ^? let value: number // $ExpectError value = "over nine thousand"; // $ExpectType number 9001; // $ExpectTypeSnapshot ["number"] type X = 1;

Debug

Known issues

breakingv0.6.0 adds versionsToTest option but may require updating configuration if you used the old undocumented API.

fix

Migrate to explicit versionsToTest array in plugin options if needed.

affects: >=0.6.0

gotchaThe plugin relies on TypeScript's language service and can be memory-intensive for large projects; a heap cache limit was introduced in v0.6.1.

fix

Upgrade to >=0.6.1 to avoid potential out-of-memory crashes.

affects: <0.6.1

gotchaCommonJS users must use require('...').default when importing the configs/recommended path.

fix

Use const expectType = require('eslint-plugin-expect-type/configs/recommended').default;

affects: >=0.4.0

deprecatedThe legacy ESLint config format (plugin:expect-type/recommended) is deprecated in favor of flat config.

fix

Switch to flat configuration using import expectType from 'eslint-plugin-expect-type/configs/recommended'.

affects: >=0.6.0

gotcha$ExpectType and $ExpectError comments must be on the line before the expression or statement, not on the same line.

fix

Place the comment on its own line immediately above the line to be checked.

affects: >=0.1.0

gotchaThe ^? Twoslash hover type comment must be placed directly after the variable/expression, with exactly one space between the variable and //.

fix

Write: let value = 9001; //  ^? let value: number

affects: >=0.1.0

Errors

Common errors & fixes

Error: Failed to load plugin 'expect-type' declared in 'plugins': Cannot find module 'eslint-plugin-expect-type'

Plugin is not installed in the project's node_modules.

fix

npm install eslint-plugin-expect-type --save-dev

TypeError: expectType is not a function

Using require() on configs/recommended without .default in CommonJS.

fix

Use require('eslint-plugin-expect-type/configs/recommended').default

ESLint: Unexpected $ExpectType comment (expect-type/expect)

The $ExpectType comment is used but the expect rule is not enabled or the comment syntax is incorrect.

fix

Enable 'expect-type/expect' rule in ESLint config and ensure the comment is on its own line before the expression.

Parsing error: Invalid or unexpected token

TypeScript syntax used in a .js file without @typescript-eslint/parser configured.

fix

Set parser: '@typescript-eslint/parser' in ESLint config for TypeScript files.

Upgrade

Version history

0.6.2latest on npm

Audit

Dependencies

@typescript-eslint/parserrequiredRequired peer dependency for parsing TypeScript with type information.

eslintrequiredRequired peer dependency – the plugin runs inside ESLint.

typescriptrequiredRequired peer dependency for type checking and resolving type assertions.

Agent activity

2 hits · last 30 days

node

Resources

githubhttps://github.com/JoshuaKGoldberg/eslint-plugin-expect-type ↗

packagehttps://www.npmjs.com/package/eslint-plugin-expect-type ↗