Add type definition for Math.sumPrecise (ES2025 / TC39 proposal)

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Bug report

📝 Description

Math.sumPrecise is part of the TC39 proposal-math-sum which reached Stage 4 on 2025-07-28, making it part of ES2026 (not ES2025 as originally stated — the proposal missed the ES2025 cutoff by about one month). Major runtimes like Bun, Firefox 137+, and Safari 18.4 have already implemented it, though V8/Node.js has not yet.

However, TypeScript does not include type definitions for this API — searching through all lib.es*.d.ts files yields no results for sumPrecise.

🐛 The problem

This causes a compile error:

Property 'sumPrecise' does not exist on type 'Math'. ts(2339)

Users are forced to either:

• Use workarounds like type augmentation (global.d.ts)
• Fall back to @ts-expect-error
• Use incorrect alternatives like Array.reduce((a, b) => a + b) which lose precision

✅ Expected behavior

The interface Math should be extended in lib.esnext.math.d.ts:

interface Math {
    /**
     * Returns the sum of the values in the iterable using a more precise
     * summation algorithm than naive floating-point addition.
     * Returns `-0` if the iterable is empty.
     * @param numbers An iterable (such as an Array) of numbers.
     * @throws {TypeError} If `numbers` is not iterable, or if any value in the iterable is not of type `number`.
     * @throws {RangeError} If the iterable yields 2^53 or more values.
     */
    sumPrecise(numbers: Iterable<number>): number;
}

🔗 Related

• TC39 Proposal: https://github.com/tc39/proposal-math-sum
• Stage 4 meeting notes: https://github.com/tc39/notes/blob/main/meetings/2025-07/july-28.md#mathsumprecise-for-stage-4
• MDN Docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sumPrecise
• Firefox 137+: shipped
• Safari 18.4 (JavaScriptCore): shipped
• Bun: shipped
• V8 / Node.js / Chrome: not yet

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