swc-project/swc · error

unknown key: {k}

Error message

unknown key: {k}

What it means

Runtime panic from swc's preset_env_base. `BrowserData` is a fixed, allocation-free struct map with a closed set of keys (chrome, chrome-android, firefox-android, opera-android, quest, react-native, and_chr, and_ff, op_mob, ie, edge, firefox, safari, node, ios, samsung, opera, android, electron, phantom, opera_mobile, rhino, deno, hermes, oculus, bun). `insert` linearly scans those keys and panics with `unknown key: {k}` for anything else — typically a browserslist query result naming an unmapped browser.

Source

Thrown at crates/preset_env_base/src/lib.rs:86

    pub hermes: T,
    #[serde(default)]
    pub oculus: T,
    #[serde(default)]
    pub bun: T,
}

impl<T> BrowserData<T>
where
    T: Default,
{
    pub fn insert(&mut self, k: &str, v: T) -> T {
        for (key, value) in self.iter_mut() {
            if k == key {
                return std::mem::replace(value, v);
            }
        }

        panic!("unknown key: {k}")
    }
}

/// A map of browser names to data for feature support in browser.
///
/// This type mainly stores `minimum version for each browsers with support for
/// a feature`.
pub type Versions = BrowserData<Option<Version>>;

impl BrowserData<Option<Version>> {
    /// Returns true if all fields are [None].
    pub fn is_any_target(&self) -> bool {
        self.iter().all(|(_, v)| v.is_none())
    }

    /// Parses the value returned from `browserslist` as [Versions].
    pub fn parse_versions(distribs: Vec<browserslist::Distrib>) -> Result<Self, Error> {
        fn remap(key: &str) -> &str {

View on GitHub (pinned to 5176682b65)

Solutions

1. Narrow the browserslist query (e.g. explicit `chrome >= 90, firefox >= 100` instead of `defaults`/percentile queries)
2. Pin the browserslist/caniuse database version so new browser ids cannot appear
3. Map or filter the unsupported key before insert (see validation below)
4. Upstream: extend BrowserData with the new browser field and its serde rename

Example fix

// before
let query = Query::from_str("> 0.5%", ...); // may include op_mini/kaios -> panic on insert

// after
let query = Query::from_str("chrome >= 90, firefox >= 100", ...); // only mapped keys

const KNOWN: &[&str] = &[
    "chrome", "chrome-android", "firefox-android", "opera-android", "quest",
    "react-native", "and_chr", "and_ff", "op_mob", "ie", "edge", "firefox",
    "safari", "node", "ios", "samsung", "opera", "android", "electron",
    "phantom", "opera_mobile", "rhino", "deno", "hermes", "oculus", "bun",
];

fn known_key(k: &str) -> bool {
    KNOWN.contains(&k)
}

// before inserting a browserslist result:
let value = if known_key(name) {
    data.insert(name, version)
} else {
    // skip, log, or map the unsupported browser instead of panicking
    log::warn!("ignoring unsupported browser target: {name}");
    Default::default()
};

fn is_supported_browser(name: &str) -> bool {
    matches!(
        name,
        "chrome" | "chrome-android" | "firefox-android" | "opera-android" | "quest"
            | "react-native" | "and_chr" | "and_ff" | "op_mob" | "ie" | "edge"
            | "firefox" | "safari" | "node" | "ios" | "samsung" | "opera" | "android"
            | "electron" | "phantom" | "opera_mobile" | "rhino" | "deno" | "hermes"
            | "oculus" | "bun"
    )
}

When it happens

Trigger: A browserslist query (e.g. `defaults`, `> 0.5%`, `cover 95%`) resolves to a browser id that has no field, such as `op_mini`, `kaios`, `and_qq`, `and_uc`, or `baidu`; query.rs then calls `BrowserData::insert` with that name and panics.

Common situations: A browserslist/caniuse database update introduces or surfaces browser ids the static map does not know; using broad market-share queries in swc preset-env / parcel-css style tooling that pull in regional browsers.

Related errors

• Invalid attempt to iterate non-iterable instance. In order t
• Duplicated methods (${element.key}) can't be decorated.
• Decorators can't be placed on different accessors with for t
• Duplicated element (${element.key})
• Duplicated element (${elements[j].key})

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/b99616268cd46bc6. Report an issue: GitHub.