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Rslint

    prefer-to-be

    Configuration

    rslint.config.ts
    import { defineConfig, jestPlugin } from '@rslint/core';

export default defineConfig([
  jestPlugin.configs.recommended,
  {
    rules: {
      'jest/prefer-to-be': 'error',
    },
  },
]);

    Rule Details

    This rule promotes the use of the most appropriate equality matcher in expect assertions, which results in more idiomatic tests and clearer failure output:

    • For primitive literals (numbers, strings, booleans), prefer toBe() over toEqual() or toStrictEqual(). The matchers behave identically here, but toBe() reads more naturally.
    • For null, undefined, and NaN, prefer the dedicated matchers toBeNull(), toBeUndefined() / toBeDefined(), and toBeNaN(). They produce more descriptive error messages than toBe(), toEqual(), or toStrictEqual().

    The rule is reported through the following messages:

    Message IDDescription
    useToBeUse toBe when expecting primitive literals
    useToBeUndefinedUse toBeUndefined instead
    useToBeDefinedUse toBeDefined instead
    useToBeNullUse toBeNull instead
    useToBeNaNUse toBeNaN instead

    Examples of incorrect code for this rule:

    // Use `toBe` for primitive literals
expect(value).not.toEqual(5);
expect(getMessage()).toStrictEqual('hello world');
expect(loadMessage()).resolves.toEqual('hello world');

// Use the dedicated matchers for `null` / `undefined` / `NaN`
expect(value).not.toBe(undefined);
expect(getMessage()).toBe(null);
expect(countMessages()).resolves.not.toBe(NaN);

    Examples of correct code for this rule:

    expect(value).not.toBe(5);
expect(getMessage()).toBe('hello world');
expect(loadMessage()).resolves.toBe('hello world');
expect(didError).not.toBe(true);

expect(value).toBeDefined();
expect(getMessage()).toBeNull();
expect(countMessages()).resolves.not.toBeNaN();

// Object/array literals still use `toStrictEqual`
expect(catchError()).toStrictEqual({ message: 'oh noes!' });
expect(catchError()).toStrictEqual({ message: undefined });

    Original Documentation