Prisma's Decimal is incompatible with decimal.js

Bug description

Prisma client bundles decimal.js and exports the class Decimal. Unfortunately, that class cannot be used with the Decimal class imported directly from decimal.js, because both classes declare toStringTag.

Example:

import { Decimal } from "decimal.js"
import { Prisma } from "@prisma/client"

const test = new Decimal(new Prisma.Decimal("12.0"))
// FYI: const test2 = new Prisma.Decimal(new Decimal("12.0")) also gives a type error

Gives the following error

Argument of type ‘Decimal’ is not assignable to parameter of type ‘Value’. Type ‘import(“*/node_modules/@prisma/client/runtime/index").Decimal’ is not assignable to type 'import("/node_modules/decimal.js/decimal”).default’. Types have separate declarations of a private property ‘toStringTag’.

How to reproduce

1. Install decimal.js 10.4.2 and @prisma/client 4.6.1.
2. The following gives a type error in typescript (tested with 4.8.4):

import { Decimal } from "decimal.js"
import { Prisma } from "@prisma/client"

const test = new Decimal(new Prisma.Decimal(12.0))
// FYI: const test2 = new Prisma.Decimal(new Decimal("12.0")) also gives a type error

Expected behavior

The Decimals from Prisma and decimal.js should be compatible.

This issue is particularly painful when working with mathjs (which depends on decimal.js). The BigNumber type from mathjs (which is just reexported Decimal from decimal.js, is also incompatible with the Prisma’s Decimal

Environment & setup

• OS: macOS Ventura
• Database: PostgreSQL
• Node.js version: 16.14

Prisma information

model Order {
  id     Int          @id @default(autoincrement())
  amount    Decimal
}

Prisma Version

prisma                  : 4.6.1
@prisma/client          : 4.6.1
Current platform        : darwin-arm64
Query Engine (Node-API) : libquery-engine 694eea289a8462c80264df36757e4fdc129b1b32 (at ../../node_modules/@prisma/engines/libquery_engine-darwin-arm64.dylib.node)
Migration Engine        : migration-engine-cli 694eea289a8462c80264df36757e4fdc129b1b32 (at ../../node_modules/@prisma/engines/migration-engine-darwin-arm64)
Introspection Engine    : introspection-core 694eea289a8462c80264df36757e4fdc129b1b32 (at ../../node_modules/@prisma/engines/introspection-engine-darwin-arm64)
Format Binary           : prisma-fmt 694eea289a8462c80264df36757e4fdc129b1b32 (at ../../node_modules/@prisma/engines/prisma-fmt-darwin-arm64)
Format Wasm             : @prisma/prisma-fmt-wasm 4.6.1-3.694eea289a8462c80264df36757e4fdc129b1b32
Default Engines Hash    : 694eea289a8462c80264df36757e4fdc129b1b32
Studio                  : 0.476.0
Preview Features        : interactiveTransactions, metrics

[jon-ressio]

Decimal.js 10.4.3 includes the PR removing toStringTag from the typescript type definition. If Prisma can update their Decimal.js dependency and you can update the direct Decimal.js dependency, that should resolve this issue.

[lorenz-h]

I’m running into this same issue. My setup is a nextjs app, prisma -> trpc -> superjson -> react.

I’m registering a custom converter for Decimals with superjson and have added a Decimal.js package dependency (10.4.2 and 10.4.1 behave the same)

I initially tried using Decimal from my React components as:

import { Decimal } from "@prisma/client/runtime";

but the prisma client/runtime package seems to reference the ‘fs’ package so I get this error in the browser:

Module not found: Can't resolve 'fs'

It would be nice if the Decimal.js bundled with Prisma could be exported cleanly (from “@prisma/client/model” or whatever makes sense) so it can be imported from browser-side code. Alternatively, if Decimal.js removes toStringTag from their typescript declaration then this problem goes away.

@jon-ressio thanks for putting together the PR - Regarding your (I believe unrelated) import issue. Have you considered importing Decimal using:

import { Prisma } from "@prisma/client"

const d: Prisma.Decimal = new Prisma.Decimal(12)

However, the duplicate declaration of toStringTag leading to incompatibility will still persist.