swc-project/swc · error

invalid utf8 character detected

Error message

invalid utf8 character detected

What it means

In swc_compiler_base's print function, the emitted JavaScript output buffer contained invalid UTF-8 when converted with String::from_utf8 after printing. JavaScript strings are UTF-16, so codegen can produce byte sequences (e.g. from lone surrogates in string literals) that are valid JS source but not valid UTF-8; this fires when the host demands a Rust String and such bytes are present.

Source

Thrown at crates/swc_compiler_base/src/lib.rs:238

            w.preamble(preamble).unwrap();
            let mut wr = Box::new(w) as Box<dyn WriteJs>;

            if codegen_config.minify {
                wr = Box::new(swc_ecma_codegen::text_writer::omit_trailing_semi(wr));
            }

            let mut emitter = Emitter {
                cfg: codegen_config,
                comments,
                cm: cm.clone(),
                wr,
            };

            node.emit_with(&mut emitter)
                .context("failed to emit module")?;
        }
        // Invalid utf8 is valid in javascript world.
        String::from_utf8(buf).expect("invalid utf8 character detected")
    };

    if cfg!(debug_assertions)
        && !src_map_buf.is_empty()
        && src_map_buf.iter().all(|(bp, _)| bp.is_dummy())
        && src.lines().count() >= 3
        && option_env!("SWC_DEBUG") == Some("1")
    {
        panic!("The module contains only dummy spans\n{src}");
    }

    let additional_scope_names = scope_buf
        .as_deref()
        .map(source_map_scopes::collect_additional_names)
        .unwrap_or_default();

    let mut map = if source_map.enabled() {
        Some(cm.build_source_map(

View on GitHub (pinned to 5176682b65)

Solutions

1. Avoid lone surrogates (\uD800-\uDFFF unpaired) in source string literals
2. Sanitize or replace unpaired surrogates in input data before compilation
3. If the output crosses a UTF-16 boundary (e.g. JS host), use lossy conversion or byte-level output APIs

When it happens

Trigger: Thrown at crates/swc_compiler_base/src/lib.rs:238 when the library encounters an invalid state.

Common situations: See trigger scenarios.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/7152b0f31f3a0432. Report an issue: GitHub.