Expect to throw with Typescript using Bun:test - ts(2348)

typescript, bunjs

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Basic Example

• Be careful on () =>

// someFunction.ts
export function someFunction(shouldThrow: boolean): void {
	if (shouldThrow) {
		throw new Error("I am an error!")
	}
}

// someFunction.test.ts
import { someFunction } from "./someFunction"

test("should throw an error", () => {
	expect(() => someFunction(true)).toThrow("I am an error!")
})

test("should not throw an error", () => {
	expect(() => someFunction(false)).not.toThrow()
})

Async Example

• Use rejects for Promise

it("should return NotFoundError", () => {
	expect(getUserData("nonexistentUserId")).rejects.toBeInstanceOf(NotFoundError)
})

Advanced Usage with Custom Error Types

• Use new CustomError() instead CustomError to prevent warning

• Be careful on () =>

// CustomError.ts
export class CustomError extends Error {
	constructor(message: string) {
		super(message)
		Object.setPrototypeOf(this, CustomError.prototype)
	}
}

// someFunction.ts
import { CustomError } from "./CustomError"

export function someFunction(shouldThrow: boolean): void {
	if (shouldThrow) {
		throw new CustomError("I am a custom error!")
	}
}

// someFunction.test.ts
import { someFunction } from "./someFunction"
import { CustomError } from "./CustomError"

test("should throw a custom error", () => {
	expect(() => someFunction(true)).toThrow(new CustomError())
	// Use `new CustomError()` instead `CustomError` to prevent warning
	// Be careful on `() =>`
})

Value of type "typeof X' is not callable. Did you mean to include 'new'? ts(2348)

• To prevent the error shown (it can run as expected) use new AlreadyAdjusted()

export class AlreadyAdjusted extends Error {
	constructor() {
		super("Already adjusted")
	}
}

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