Welcome back! Let's go back on track after bonus questions. This question will be less abstract, and more practical, we will land more at value level 🚁.

We have a function concatToField which takes object, and key of this object, and string value. The purpose of this function is to create a new object with concatenated property object[key] with third argument.

The question - How to type generic types T and K in the definition of concatToField function in order to achieve compile time guarantee that obj[key] can be only string.

const concatToField =
  <T /* here your code 💪*/, K /* here your code 💪*/>(obj: T, key: K, payload: string): T => {
    const prop = obj[key]; // compile error should not be here
    return { ...obj, [key]: prop.concat(payload) }; // compile error should not be here
}
// tests
const test = { fieldStr: 'text', fieldNum: 1, fieldStr2: 'text' };
concatToField(test, 'fieldStr', 'test'); // should be ok 👌
concatToField(test, 'fieldNum', 'test'); // should be error fieldNum is not string field 🛑
concatToField(test, 'notExistingField', 'test'); // should be error - no such field 🛑
concatToField(test, 'fieldStr2', 'test'); // should be ok 👌

Full code available in the playground

Important - body of the function should remain unchanged, no type assertion (as), or any changes of the body are allowed. The only thing needs to be done is constraint on T and K generic types.

Post your answers in comments (preferred links to the playground). Have fun! Answer will be published soon!

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