prefer-to-be.md

Suggest using toBe() for primitive literals (prefer-to-be)

When asserting against primitive literals such as numbers and strings, the equality matchers all operate the same, but read slightly differently in code.

This rule recommends using the toBe matcher in these situations, as it forms the most grammatically natural sentence. For null, undefined, and NaN this rule recommends using their specific toBe matchers, as they give better error messages as well.

Rule details

This rule triggers a warning if toEqual() or toStrictEqual() are used to assert a primitive literal value such as numbers, strings, and booleans.

The following patterns are considered warnings:

expect(value).not.toEqual(5);
expect(getMessage()).toStrictEqual('hello world');
expect(loadMessage()).resolves.toEqual('hello world');

The following pattern is not warning:

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

expect(catchError()).toStrictEqual({ message: 'oh noes!' });

For null, undefined, and NaN, this rule triggers a warning if toBe is used to assert against those literal values instead of their more specific toBe counterparts:

expect(value).not.toBe(undefined);
expect(getMessage()).toBe(null);
expect(countMessages()).resolves.not.toBe(NaN);

The following pattern is not warning:

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

expect(catchError()).toStrictEqual({ message: undefined });