oxc-project/oxc · warning

Suggest using `toContain()`.

Error message

Suggest using `toContain()`.

What it means

Warning from the oxlint `jest/prefer-to-contain` rule (source: crates/oxc_linter/src/rules/shared/jest_vitest/prefer_to_contain.rs:18). The rule inspects `expect()` calls whose argument is an `includes(...)` call and whose matcher is an equality matcher (`toBe`/`toEqual`/`toStrictEqual`) with a boolean literal; it recommends `toContain()`, whose failure output shows the actual array contents.

Source

Thrown at crates/oxc_linter/src/rules/shared/jest_vitest/prefer_to_contain.rs:18

use oxc_ast::{
    AstKind,
    ast::{Argument, CallExpression, Expression},
};
use oxc_diagnostics::OxcDiagnostic;
use oxc_ecmascript::{ToBoolean, WithoutGlobalReferenceInformation};
use oxc_span::Span;

use crate::{
    context::LintContext,
    utils::{
        KnownMemberExpressionParentKind, PossibleJestNode, is_equality_matcher,
        parse_expect_jest_fn_call,
    },
};

fn use_to_contain(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Suggest using `toContain()`.").with_label(span)
}

pub const DOCUMENTATION: &str = r"### What it does

In order to have a better failure message, `toContain()` should be used upon
asserting expectations on an array containing an object.

### Why is this bad?

This rule triggers a warning if `toBe()`, `toEqual()` or `toStrictEqual()` is
used to assert object inclusion in an array

### Examples

Examples of **incorrect** code for this rule:
```javascript
expect(a.includes(b)).toBe(true);
expect(a.includes(b)).not.toBe(true);

View on GitHub (pinned to e1e7af627c)

Solutions

1. Rewrite the assertion to move the array into `expect` and use `toContain`: `expect(arr).toContain(item)`.
2. For negative membership use `expect(arr).not.toContain(item)`.
3. Suppress inline with `// oxlint-disable-next-line jest/prefer-to-contain` when asserting on the boolean itself is the point (e.g. testing an `includes` polyfill).

Example fix

// before
expect(allowedUsers.includes('alice')).toBe(true);

// after
expect(allowedUsers).toContain('alice');

// rg -n "\.includes\([^)]*\)\)\.(toBe|toEqual|toStrictEqual)\((true|false)\)" tests/ -t ts -t js

When it happens

Trigger: `expect(arr.includes(item)).toBe(true)`, `expect(arr.includes(item)).toEqual(false)`, or `expect(list.includes(x)).not.toBe(true)` - i.e. an equality matcher applied to the boolean result of `Array.prototype.includes`. The implementation uses `oxc_ecmascript::ToBoolean` (with `WithoutGlobalReferenceInformation`) to recognize the boolean-literal argument.

Common situations: Developers unfamiliar with `toContain` asserting membership the manual way; refactors of loops that used `indexOf(...) !== -1`; CI running oxlint with the jest plugin after migration from ESLint.

Related errors

• Suggest using `toStrictEqual()`.
• Use `toBe` when expecting primitive literals.
• Use `toBeUndefined` instead.
• Use `toBeDefined` instead.
• Use `toBeNull` instead.

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/07606644c22b4d5a. Report an issue: GitHub.