oxc-project/oxc · warning · OxcDiagnostic

Unexpected hexadecimal digits in lowercase.

Error message

Unexpected hexadecimal digits in lowercase.

What it means

Lint diagnostic from oxlint's `unicorn/number-literal-case` rule. Unlike the prefix and exponent checks, hex digits are required to be UPPERCASE for readability, so `0xff` is flagged and `0xFF` is the canonical form. This variant fires when hexadecimal digits are lowercase while the prefix itself is already correct (`0x`).

Source

Thrown at crates/oxc_linter/src/rules/unicorn/number_literal_case.rs:22

use oxc_macros::declare_oxc_lint;
use oxc_span::Span;

use crate::{AstNode, context::LintContext, rule::Rule};

fn uppercase_prefix(span: Span, prefix: &str) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected number literal prefix in uppercase.")
        .with_help(format!("Use lowercase for the number literal prefix `{prefix}`."))
        .with_label(span)
}

fn uppercase_exponential_notation(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected exponential notation in uppercase.")
        .with_help("Use lowercase for `e` in exponential notations.")
        .with_label(span)
}

fn lowercase_hexadecimal_digits(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected hexadecimal digits in lowercase.")
        .with_help("Use uppercase for hexadecimal digits.")
        .with_label(span)
}

fn uppercase_prefix_and_lowercase_hexadecimal_digits(span: Span, prefix: &str) -> OxcDiagnostic {
    OxcDiagnostic::warn(
        "Unexpected number literal prefix in uppercase and hexadecimal digits in lowercase.",
    )
    .with_help(format!(
        "Use lowercase for the number literal prefix `{prefix}` and uppercase for hexadecimal digits."
    ))
    .with_label(span)
}

#[derive(Debug, Default, Clone)]
pub struct NumberLiteralCase;

declare_oxc_lint!(

View on GitHub (pinned to e1e7af627c)

Solutions

1. Uppercase the hex digits: `0xff0000` -> `0xFF0000`.
2. Run `oxlint --fix` to rewrite all lowercase hex digits repo-wide.
3. For generated files, exclude the path in `.oxlintrc.json` `ignorePatterns` instead of hand-editing.
4. Disable the rule with `"unicorn/number-literal-case": "off"` if lowercase hex is a deliberate project convention.

Example fix

// before
const RED = 0xff0000;
const MASK = 0xdead_beef;

// after
const RED = 0xFF0000;
const MASK = 0xDEAD_BEEF;

// oxlint --fix src/constants/colors.ts
// Verify no remaining lowercase-hex violations:
// oxlint --deny-warnings unicorn/number-literal-case

When it happens

Trigger: Hex literals with lowercase a-f digits: `const red = 0xff0000;`, `let mask = 0xdeadbeef;`. Triggered while oxlint scans files with the unicorn plugin enabled.

Common situations: Color constants, bitmask flags, and hashes are commonly written lowercase (CSS habit, `#ff0000`). Teams adopting oxlint's unicorn category see this on color/flag tables. Also appears in vendored or machine-generated code that was never normalized.

Related errors

• Unexpected number literal prefix in uppercase and hexadecima
• Don't use a zero fraction in the number.
• Don't use a dangling dot in the number.
• Unexpected number literal prefix in uppercase.
• Unexpected exponential notation in uppercase.

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/3106b75aa548542c. Report an issue: GitHub.