The JS package

npm install costql gives you the quote side of costQL in JavaScript/TypeScript: load the same pack file the Python engine writes and price queries offline in Node, browsers, or edge runtimes. Zero runtime dependencies. Building packs stays in Python (pip install 'costql[build]') by design: that side needs a live API and numerical fitting; quoting is pure traversal and belongs everywhere your app runs.

Usage

import { PricingPack, validate } from "costql";

// Node
const pack = await PricingPack.load("packs/tmdb_t3.json");

// Browser / edge: fetch the pack like any static asset
const pack = PricingPack.fromObject(await (await fetch("/packs/tmdb_t3.json")).json());

const quote = pack.quote('{ movie(id:"27205"){ cast(limit:8){ person{ name } } } }');
quote.price           // 50.5384 (safe max, cost-units)
quote.typical_price   // 36.3769 (typical everyday cost)
quote.confidence      // "high" | "medium" | "low"
quote.caveats         // e.g. the cyclic-recursion explanation, when flagged
quote.breakdown       // per-resolver cost lines (T2/T3 packs)
quote.sharing         // folded shared loaders (T3 packs)
quote.external_calls  // named outside hosts + call count (T3 packs)

validate(quote)       // [] (contract v1.0 violations, if any)

The result object is the frozen output contract v1.0: the identical shape the Python engine emits, tier-gating included.

PricingPack.fromObject / fromJSON throw PackVersionError on a pack written by a newer costql, a missing section, or a non-pack, the same gate as the Python loader.

The conformance guarantee

“Identical to the Python engine” is a tested claim, not a hope. The repo freezes an oracle (conformance/quotes.json, 56 full quote results spanning simple, fanout, sharing-heavy, external-call, cyclic, and sugared (fragments/aliases/variables) queries across the five demo packs plus a synthetic union-schema pack) and CI runs both engines against it on every PR. Numbers must agree within max(1e-6, 1e-9 · |expected|); every other field must be deep-equal. Even the caveat strings match.

Pinning: costql@0.1.x (npm) conforms to costql 0.1.x (PyPI), contract v1.0, pack_version 1.

Shared parser, shared behavior

The JS parser is a deliberate line-for-line port of the Python one, so the two engines accept the same query surface (fragments, aliases, variables, directives: see Quoting queries) and price it identically. To pass variable values, hand them to quote directly:

pack.quote('query($n: Int!){ movie(id:"27205"){ cast(limit:$n){ person{ name } } } }',
           { n: 4 });

If you hit an edge where the two engines disagree, that is a bug. Please file it with the pack + query.

What’s deliberately absent

• No build/calibration APIs: Python-only.
• No network access in quote(): a pack in, a contract result out.
• No dollars: cost-units only; currency conversion is your app’s concern.