Modernize tooling: Migrate TSLint→ESLint, TypeScript 5.3, custom error classes

[Copilot]

TSLint has been deprecated since 2019. This PR migrates to ESLint, upgrades TypeScript to 5.3.3, and modernizes the codebase to 2026 standards while maintaining 100% backward compatibility.

Tooling Migration

• TSLint → ESLint 8.56: Removed deprecated TSLint dependencies, added @typescript-eslint plugins with type-aware rules
• TypeScript 4.2 → 5.3: Updated target to ES2020, enabled stricter checks (noUnusedLocals, noUnusedParameters, noImplicitReturns, noFallthroughCasesInSwitch)
• Build scripts: Removed hacky postbuild sed script, added type-check and prepublishOnly validation

Code Quality

• Removed 14 @ts-ignore suppressions: Fixed with proper type assertions (as unknown as T, type guards) or documented @ts-expect-error where structurally necessary (ReadOnlyDictionary static method incompatibility)
• Replaced tslint-disable comments: Migrated to equivalent ESLint suppressions (@typescript-eslint/no-explicit-any, @typescript-eslint/no-unsafe-call)

Error Handling

Created typed error class hierarchy extending from CollectionError:

// Before: generic Error objects
throw new Error('Sequence contains no elements');

// After: typed error classes
throw new NoElementsError();

// Backward compatible via deprecated Errors class
Errors.noElements(); // Still works, returns NoElementsError instance

New error classes exported: CollectionError, ArgumentOutOfRangeError, IndexOutOfRangeError, DuplicateKeyError, NoElementsError, NoMatchError, etc.

Note: ValueNotNumberError extends TypeError (not CollectionError) to match original implementation behavior.

Configuration

Added .editorconfig for consistent formatting across editors.

Original prompt

Objective

Modernize the sharp-collections codebase to align with current TypeScript, JavaScript, and tooling best practices as of 2026.

Critical Changes

1. Replace TSLint with ESLint (CRITICAL - TSLint deprecated in 2019)

Remove these dependencies from package.json:

"tslint": "~6.1.0",
"tslint-eslint-rules": "~5.4.0",
"tslint-microsoft-contrib": "~6.2.0",
"tslint-no-unused-expression-chai": "^0.1.4",
"tslint-teamcity-reporter": "~3.2.2"

Add ESLint dependencies:

"@typescript-eslint/eslint-plugin": "^6.21.0",
"@typescript-eslint/parser": "^6.21.0",
"eslint": "^8.56.0",
"eslint-plugin-chai-friendly": "^0.7.4"

Create .eslintrc.json:

{
  "root": true,
  "parser": "@typescript-eslint/parser",
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/recommended",
    "plugin:@typescript-eslint/recommended-requiring-type-checking"
  ],
  "plugins": ["@typescript-eslint", "chai-friendly"],
  "parserOptions": {
    "ecmaVersion": 2022,
    "sourceType": "module",
    "project": "./tsconfig.json"
  },
  "rules": {
    "no-unused-expressions": "off",
    "chai-friendly/no-unused-expressions": "error",
    "@typescript-eslint/no-explicit-any": "warn"
  },
  "ignorePatterns": ["dist/", "node_modules/", "*.js"]
}

Create .eslintignore:

dist/
node_modules/
*.js
coverage/

Delete tslint.json file

Update package.json scripts:

"lint": "eslint src/**/*.ts tests/**/*.ts",
"lint:fix": "eslint src/**/*.ts tests/**/*.ts --fix"

2. Remove All @ts-ignore Comments

In src/extensions/enumerable/Single.ts:
Replace:

function single<T>(this: Enumerable<T>, predicate?: (x: T, index: number) => boolean): T {
    let value: T;
    let found = false;
    // ...
    // @ts-ignore
    return value;
}

With:

function single<T>(this: Enumerable<T>, predicate?: (x: T, index: number) => boolean): T {
    let value: T | undefined;
    let found = false;
    let index = 0;
    for (const element of this) {
        if (predicate == null || predicate(element, index++)) {
            if (found) {
                throw predicate != null ? Errors.moreThanOneMatch() : Errors.moreThanOneElement();
            }
            value = element;
            found = true;
        }
    }

    if (!found || value === undefined) {
        throw predicate != null ? Errors.noMatch() : Errors.noElements();
    }

    return value;
}

Find and fix any other @ts-ignore comments in the codebase

3. Update TypeScript Configuration

Update tsconfig.json:

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "ES2020",
    "lib": ["ES2020", "ES2021", "ES2022"],
    "moduleResolution": "bundler",
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "outDir": "./dist",
    "downlevelIteration": true,
    "strict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "baseUrl": "./",
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "exactOptionalPropertyTypes": true,
    "noUncheckedIndexedAccess": true,
    "removeComments": false
  },
  "include": ["src"],
  "exclude": ["node_modules", "**/__tests__/*", "dist"]
}

4. Modernize Error Handling with Custom Error Classes

Replace src/Errors.ts content:

/**
 * Base error class for all collection-related errors
 */
export class CollectionError extends Error {
    constructor(message: string) {
        super(message);
        this.name = 'CollectionError';
        Object.setPrototypeOf(this, CollectionError.prototype);
    }
}

/**
 * Thrown when an argument is out of range
 */
export class ArgumentOutOfRangeError extends CollectionError {
    constructor(paramName?: string) {
        super(`Argument${paramName ? ` '${paramName}'` : ''} was out of range`);
        this.name = 'ArgumentOutOfRangeError';
    }
}

/**
 * Thrown when an index is out of range
 */
export class IndexOutOfRangeError extends CollectionError {
    constructor() {
        super('Index was out of range');
        this.name = 'IndexOutOfRangeError';
    }
}

/**
 * Thrown when an index is not an integer
 */
export class IndexNotIntegerError extends CollectionError {
    constructor() {
        super('Index was not an integer');
        this.name = 'IndexNotIntegerError';
    }
}

/**
 * Thrown when an item with the same key has already been added
 */
export class DuplicateKeyError extends CollectionError {
    constructor() {
        super('An item with the same key has already been added');
        this.name = 'DuplicateKeyError';
    }
}

/**
 * Thrown when an element has already been added to a set
 */
export class DuplicateElementError extends CollectionError {
    constructor() {
        super('The element has already been added');
        this.name = 'DuplicateElementError';
    ...

</details>



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