sharils-try

Problems

Let's look at the following snippet.

let data;
try {
    data = JSOM.parse(json);
} catch (e) {
    console.error("syntax error in the json string");
}
console.log(data);

This snippet has three problems:

1. The console is printed "syntax error in the json string" however, it's triggered by the JSOM typo because a ReferenceError can be caught.
2. data has to be declared using let because try is a block
3. e is not available outside of the catch clause.

There is a awakard way to fix problem 1. See below:

let data;
try {
    data = JSOM.parse(json);
} catch (e) {
    if (e.constructor === ReferenceError) {
        console.error("syntax error in the json string");
    }
    throw e;
}

Solution

const [e, data] = tryCall([ReferenceError], JSON.parse, json);
if (e) {
    console.error("syntax error in the json string");
} else {
    console.log(data);
}

This returns a sparse array for easy destructuring, and only one of the element will be set depending on the index of the error class if fail or set to the last index if succeed.

Philosophy

The reason errors exist before the call result is because this library thinks all errors should be handled, thus it's designed to make ignoring errors difficult. You can't easily get result without getting all errors.

The reason there is no flag to tell if there is no error thrown is also because of the same reason above.

Generator

There is tryYield to do the same thing as tryCall in a generator. Let's take the following snippet from redux-saga.

function* fetchUser(action) {
    try {
        const user = yield call(Api.fetchUser, action.payload.userId);
        yield put({type: "USER_FETCH_SUCCEEDED", user: user});
    } catch (e) {
        yield put({type: "USER_FETCH_FAILED", message: e.message});
    }
}

Using tryYield, this can be written as the following:

function* fetchUser(action) {
    const [e, user] = yield* tryYield([UserFetchError], call(Api.fetchUser, action.payload.userId));
    if (e) {
        yield put({type: "USER_FETCH_FAILED", message: e.message});
    } else {
        yield put({type: "USER_FETCH_SUCCEEDED", user: user});
    }
}

Note that yield call becomes yield* tryYield. And call(Api.fetchUser, action.payload.userId) is directly wrapped by tryYield instead of tryYield(call, Api.fetchUser, action.payload.userId).

Test

yarn test

Build

yarn run build