What causes "%s: --icu4xMode requires --ucadata\n" in nodejs/node?

Always pass --ucadata alongside --icu4xMode, pointing at the compiled collation/uca data file.

%s: --icu4xMode requires --ucadata\n

Error message

%s: --icu4xMode requires --ucadata\n

What it means

genrb's --icu4xMode produces output tailored for ICU4X consumption and depends on a precompiled uca data file (--ucadata). Using --icu4xMode without --ucadata is unsupported, so genrb records the violation via illegalArg and exits non-zero after printing usage.

Source

Thrown at deps/icu-small/source/tools/genrb/genrb.cpp:161

    U_MAIN_INIT_ARGS(argc, argv);

    options[JAVA_PACKAGE].value = "com.ibm.icu.impl.data";
    options[BUNDLE_NAME].value = "LocaleElements";
    argc = u_parseArgs(argc, argv, UPRV_LENGTHOF(options), options);

    /* error handling, printing usage message */
    if(argc<0) {
        fprintf(stderr, "%s: error in command line argument \"%s\"\n", argv[0], argv[-argc]);
        illegalArg = true;
    } else if(argc<2) {
        illegalArg = true;
    }
    if(options[WRITE_POOL_BUNDLE].doesOccur && options[USE_POOL_BUNDLE].doesOccur) {
        fprintf(stderr, "%s: cannot combine --writePoolBundle and --usePoolBundle\n", argv[0]);
        illegalArg = true;
    }
    if (options[ICU4X_MODE].doesOccur && !options[UCADATA].doesOccur) {
        fprintf(stderr, "%s: --icu4xMode requires --ucadata\n", argv[0]);
        illegalArg = true;
    }
    if(options[FORMAT_VERSION].doesOccur) {
        const char *s = options[FORMAT_VERSION].value;
        if(uprv_strlen(s) != 1 || (s[0] < '1' || '3' < s[0])) {
            fprintf(stderr, "%s: unsupported --formatVersion %s\n", argv[0], s);
            illegalArg = true;
        } else if(s[0] == '1' &&
                  (options[WRITE_POOL_BUNDLE].doesOccur || options[USE_POOL_BUNDLE].doesOccur)
        ) {
            fprintf(stderr, "%s: cannot combine --formatVersion 1 with --writePoolBundle or --usePoolBundle\n", argv[0]);
            illegalArg = true;
        } else {
            setFormatVersion(s[0] - '0');
        }
    }

    if((options[JAVA_PACKAGE].doesOccur || options[BUNDLE_NAME].doesOccur) &&

View on GitHub (pinned to 1b2de5e052)

Solutions

Always pass --ucadata <file> alongside --icu4xMode, pointing at the compiled collation/uca data file.
If you did not intend ICU4X output, remove --icu4xMode.
Wire your build so that enabling ICU4X mode also supplies the ucadata path (e.g. a single make variable that sets both).

Example fix

# before
genrb --icu4xMode en.txt

# after
genrb --icu4xMode --ucadata uca.icu en.txt

Defensive patterns

Strategy: validation

Validate before calling

// require --ucadata whenever --icu4xMode is set
if '--icu4xMode' in argv and '--ucadata' not in argv:
    raise SystemExit('genrb: --icu4xMode requires --ucadata <file>')

Prevention

Bind both flags to one build variable so enabling ICU4X mode always supplies the uca data.
Document the ucadata input path in the build config alongside the ICU4X switch.

When it happens

Trigger: options[ICU4X_MODE].doesOccur is true while options[UCADATA].doesOccur is false on the same genrb command.

Common situations: Adopting ICU4X-compatible data generation but forgetting the uca data input; copying a partial flag set from migration docs; a build target that toggles --icu4xMode via a variable without also setting --ucadata.

Related errors

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/58944ef7aa2ff8a2. Report an issue: GitHub.