oxc-project/oxc · warning

Unexpected implicit coercion to string

Error message

Unexpected implicit coercion to string

What it means

The string-kind diagnostic of oxlint's no-implicit-coercion rule. It flags template literals whose only content is a single interpolation (`${value}`), string concatenation like value + '' or '' + value, and value += ''. The rule wants String(value) so the conversion is explicit; templates that contain literal text around interpolations are fine.

Source

Thrown at crates/oxc_linter/src/rules/eslint/no_implicit_coercion.rs:29

    AstNode,
    context::LintContext,
    rule::{DefaultRuleConfig, Rule},
};

fn boolean_coercion_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected implicit coercion to boolean")
        .with_help("Use `Boolean(value)` instead")
        .with_label(span)
}

fn number_coercion_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected implicit coercion to number")
        .with_help("Use `Number(value)` instead")
        .with_label(span)
}

fn string_coercion_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected implicit coercion to string")
        .with_help("Use `String(value)` instead")
        .with_label(span)
}

/// Type of implicit coercion being detected
#[derive(Clone, Copy)]
enum CoercionKind {
    Boolean,
    Number,
    String,
}

/// Reports an implicit coercion with an auto-fix suggestion
fn report_coercion(ctx: &LintContext, span: Span, kind: CoercionKind, operand: &str) {
    let (diagnostic, wrapper) = match kind {
        CoercionKind::Boolean => (boolean_coercion_diagnostic(span), "Boolean"),
        CoercionKind::Number => (number_coercion_diagnostic(span), "Number"),
        CoercionKind::String => (string_coercion_diagnostic(span), "String"),

View on GitHub (pinned to e1e7af627c)

Solutions

1. Use String(value) for the conversion.
2. Keep template literals only when they contain literal text around the interpolation.
3. Disable string checks in the rule options if template style is intentional.

Example fix

// before
const key = `${user.id}`;

// after
const key = String(user.id);

const strCast = /(?:`\$\{[^}]+\}`|\+\s*''|''\s*\+)/.test(source);

const asStr = (v: unknown): string => (typeof v === 'string' ? v : String(v));

When it happens

Trigger: const key = `${user.id}`;; '' + flags; text += ''; concatenating x + '' to build object keys or log lines.

Common situations: Building object keys from ids; stringifying numbers or symbols for logging; the '' + x trick carried over from older snippets.

Related errors

• Unexpected implicit coercion to boolean
• Unexpected implicit coercion to number
• Unexpected string concatenation of literals.
• Unnecessary escape character {escape_char:?}
• Empty array binding pattern

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/620bb7c4ac447b9a. Report an issue: GitHub.