How to add types checker for the Jest in the TypeScript app?

Question

I need to add some type checker to the Jest. It must look somehow like expect(someVar).toBeType('string') and expect(someVar).toBeType(['string', 'object']).

I tried to add some checker-helper, but it looks a little bit ugly.

const toBeType = (arg:any, type:string) => {
    const argType = typeof arg;

    if (argType !== type)
        throw new Error(`Expected '${type}' but got '${argType}' for the '${arg}'.`);
};

I want to add similar functionality to the jest namespace to have ability to call type checker like expect(someVar).toBeType('boolean').

Answer 1

I resolved this problem this way. To add functionality to the Jest we should use expect.extend({...}). So, to add toBeType method to the Jest we should write this code to some setupTests.js file:

// setupTests.js

expect.extend({
    /**
     * @param {*} received
     * @param {string|string[]} arg
     * @return {{pass:boolean,message:(function():string)}}
     */
    toBeType(received, arg) {
        const isCorrectType = arg => {
            const receivedType = typeof received;

            const checkForSingle = arg => {
                const type = receivedType === 'object'
                    ? Array.isArray(received)
                        ? 'array'
                        : receivedType
                    : receivedType;

                return type === arg;
            };

            const checkForArr = arg => {
                const reducer = (prev, curr) => prev
                    || isCorrectType(curr).isCorrect;

                return arg.reduce(reducer, false);
            };

            return {
                receivedType,
                isCorrect: Array.isArray(arg)
                    ? checkForArr(arg)
                    : checkForSingle(arg)
            };
        };

        const {isCorrect, receivedType} = isCorrectType(arg);

        return {
            pass: isCorrect,
            message: () => {
                const toBe = Array.isArray(arg)
                    ? arg.join(`' or '`)
                    : arg;

                return `Expected '${received}' of '${receivedType}' type to be of '${toBe}' type(s)`;
            }
        };
    }
});

Don't forget to add setupTests.js to the jest.config.js file as follows:

// jest.config.js

module.exports = {
    ...your_configurations...
    setupFilesAfterEnv: ['<rootDir>/setupTests.js'],
};

Also we have to extend global.d.ts file to say interpreter we have toBeType method at the extend namespace (it is required if only you are using TypeScript). Here is code we have to add to global.d.ts:

// global.d.ts

declare namespace jest {
    interface Matchers<R> {
        toBeType(type:string|string[]);
    }
}

This code says: get jest namespace and extend Matchers<R> interface with the toBeType method. (You can look at Matchers<R> interface implementation at the @types/jest node module.)