Javascript exception handling leaves much to be asked. That is why this package provides a method for typesafe error handling. When combined with typescript type coercion you are left with a powerful primitive to signal the results of operations.
$ npm install ts-err-ok
Result declares your possible return values. In this case number if ok, else Error.
function divide(x: number, y: number): Result<number, Error>;The compiler forces you to check whether the operation was successful or not.
const result = divide(10, 5);
if (result.ok()) {
// Compiler coerces "result" into Ok<number>.
console.log("Result" + result.val);
} else {
// Compiler coerces "result" into Err<Error>.
console.log("Error" + result.error);
}
// This results in a type error because the result has to explicitly checked before either "val" or "error" can be accessed.
console.log("Result: ", result.val);Build result objects by using the helper functions ok() and err().
function divide(x: number, y: number): Result<number, Error> {
if (y === 0) {
// Use err() to construct an error result.
return err(new Error("Division by zero"));
} else {
// Use ok() to oconstruct an ok result.
return ok(x / y);
}
}By using discriminated unions as your error types you can get very fine grained error handling. An example that parses songs out of json strings
interface Song {
author: string;
title: string;
}
interface MissingField {
type: "MissingField";
message: string;
}
interface JsonSyntaxError {
type: "JsonSyntaxError";
syntaxError: SyntaxError;
}
type SongParseError = MissingField | JsonSyntaxError;
export function parseSong(str: string): Result<Song, SongParseError> {
try {
const obj = JSON.parse(str);
if (obj.author === undefined) {
return err({
type: "MissingField",
message: "Author was undefined",
});
}
if (obj.title === undefined) {
return err({
type: "MissingField",
message: "Title was undefined",
});
}
return ok(obj);
} catch (error) {
if (error instanceof SyntaxError) {
return err({
type: "JsonSyntaxError",
syntaxError: error,
});
} else {
// Rethrow unexpected errors.
throw error;
}
}
}
const res = parseSong("{[]");
if (res.ok()) {
// "val" has type "Song".
console.log("Song: ", res.val);
} else {
switch (res.error.type) {
case "MissingField":
// "error" has type "MissingField".
console.log("Missing field: ", res.error.message);
break;
case "JsonSyntaxError":
// "error" has type "SyntaxError".
console.log("Syntax error: ", res.error.syntaxError);
break;
}
}