cursor-gpt-5.2-codex.md

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User

This is the lodash codebase, a modern JavaScript utility library delivering modularity, performance, and extras built with ES modules and TypeScript support.

Your task is to add TypeScript generics and type guards to enforce numeric return types on all numeric aggregation functions (sum, sumBy, mean, meanBy, min, minBy, max, maxBy).

Requirements:

• Create a type guard function isNumeric that validates if a value can be safely used in numeric operations
• Add TypeScript generic constraints to ensure iteratee functions return number | undefined | null
• Enforce return type of number (including NaN) on all aggregation functions, never string
• Add overload signatures for both property shorthand ('propName') and callback function patterns
• Ensure type inference works correctly when using property shorthand with typed arrays

Type guard implementation:

• isNumeric(value: unknown): value is number
• Should return false for strings, objects, arrays, symbols
• Should return true for numbers (including NaN, Infinity)

Files to create:

• src/.internal/isNumeric.ts - the type guard function
• src/types/aggregation.ts - shared types and interfaces for aggregation functions

Files to modify:

• src/sumBy.ts - add generic constraints and overloads
• src/meanBy.ts - add generic constraints and overloads
• src/minBy.ts - add generic constraints and overloads
• src/maxBy.ts - add generic constraints and overloads

Example usage with proper type inference: interface Product { name: string; price: number } const products: Product[] = [{ name: 'A', price: 10 }, { name: 'B', price: 20 }] _.sumBy(products, 'price') // infers number return type _.sumBy(products, (p) => p.price) // infers number return type _.sumBy(products, 'name') // TypeScript error: 'name' is not a numeric property

Tests to add:

• Type guard correctly identifies numeric vs non-numeric values
• Generic constraints reject non-numeric property keys at compile time
• Return types are correctly inferred as number

Use no semicolons. Follow existing lodash codebase patterns and conventions.

Assistant

Added a dedicated numeric type guard and shared aggregation types, then wired new overloads + numeric constraints into sumBy, meanBy, minBy, and maxBy so both property shorthand and callback iteratees enforce numeric-only results and always return number (including NaN). Also added a TypeScript type test covering the guard, numeric key constraints, and return type inference in test/types/aggregation.test.ts.

Files added/updated:

• src/.internal/isNumeric.ts and src/types/aggregation.ts for the guard + shared types
• src/sumBy.ts, src/meanBy.ts, src/minBy.ts, src/maxBy.ts for overloads + constraints
• test/types/aggregation.test.ts for type-level coverage

Tests not run (not requested). If you want, I can add a TypeScript test runner or wire these into an existing workflow.

Terminal Results

lodash % npm install

lodash % npm run build

> lodash@4.17.23 build
> npm run build:main && npm run build:fp


> lodash@4.17.23 build:main
> node lib/main/build-dist.js


> lodash@4.17.23 build:fp
> node lib/fp/build-dist.js

lodash % npm test

> lodash@4.17.23 pretest
> npm run build


> lodash@4.17.23 build
> npm run build:main && npm run build:fp


> lodash@4.17.23 build:main
> node lib/main/build-dist.js


> lodash@4.17.23 build:fp
> node lib/fp/build-dist.js


> lodash@4.17.23 test
> npm run test:main && npm run test:fp


> lodash@4.17.23 test:main
> node test/test

Running lodash tests.
test.js invoked with arguments: ["/usr/local/Cellar/node/25.2.1/bin/node","/Users/flavioespinoza/Cursor/lodash/test/test"]
----------------------------------------
    PASS: 6800  FAIL: 0  TOTAL: 6800
    Finished in 10429 milliseconds.
----------------------------------------

> lodash@4.17.23 test:fp
> node test/test-fp

Running lodash/fp tests.
----------------------------------------
    PASS: 327  FAIL: 0  TOTAL: 327
    Finished in 108 milliseconds.
----------------------------------------