Error Handling and Validation

Result-Style Unions

A result union returns either a success value or an error message without throwing.

result union A result union uses a discriminant such as `ok` to separate success and failure paths.

Result-Style Unions

rawCount
result.ts
Replay: real traced execution (multi-file project)
type ParseResult =
    | { ok: true; value: number }
    | { ok: false; error: string };

function parseCount(text: string): ParseResult {
    const value: number = Number(text);
    if (Number.isNaN(value)) {
        return { ok: false, error: "not a number" };
    }
    return { ok: true, value: value };
}

const rawCount: string = "12";
const parsed: ParseResult = parseCount(rawCount);

if (parsed.ok) {
    console.log(`count=${parsed.value}`);
} else {
    console.log(`error=${parsed.error}`);
}

type ParseResult =
    | { ok: true; value: number }
    | { ok: false; error: string };

function parseCount(text: string): ParseResult {
    const value: number = Number(text);
    if (Number.isNaN(value)) {
        return { ok: false, error: "not a number" };
    }
    return { ok: true, value: value };
}

const rawCount: string = "x";
const parsed: ParseResult = parseCount(rawCount);

if (parsed.ok) {
    console.log(`count=${parsed.value}`);
} else {
    console.log(`error=${parsed.error}`);
}

type ParseResult =
    | { ok: true; value: number }
    | { ok: false; error: string };

function parseCount(text: string): ParseResult {
    const value: number = Number(text);
    if (Number.isNaN(value)) {
        return { ok: false, error: "not a number" };
    }
    return { ok: true, value: value };
}

const rawCount: string = "7";
const parsed: ParseResult = parseCount(rawCount);

if (parsed.ok) {
    console.log(`count=${parsed.value}`);
} else {
    console.log(`error=${parsed.error}`);
}

  1. rawCount ← 12

    10    return { ok: true, value: value };11}1213const rawCount→ 12: string = "12";  //@rawCount="x", "7"14const parsed: ParseResult = parseCount(rawCount12);

  2. value ← 12

    2    | { ok: true; value: number }3    | { ok: false; error: string };45function parseCount(text12: string): ParseResult {6    const value→ 12: number = Number(text12);7    if (Number.isNaN(value)) {8        return { ok: false, error: "not a number" };9    }10    return { ok: true, value: value12 };11}

  3. parsed ← [object Object]

    13const rawCount: string = "12";  //@rawCount="x", "7"14const parsed→ [object Object]: ParseResult = parseCount(rawCount12);

  4. if (parsed.ok)

    13const rawCount: string = "12";  //@rawCount="x", "7"14const parsed: ParseResult = parseCount(rawCount);1516if (parsed.oktrue) {17    console.log(`count=${parsed.value12}`);18} else {
    outputcount=12

  1. rawCount ← x

    10    return { ok: true, value: value };11}1213const rawCount→ x: string = "x";14const parsed: ParseResult = parseCount(rawCountx);

  2. value ← NaN

    2    | { ok: true; value: number }3    | { ok: false; error: string };45function parseCount(textx: string): ParseResult {6    const value→ NaN: number = Number(textx);7    if (Number.isNaN(value)) {

  3. if (Number.isNaN(value))

    5function parseCount(text: string): ParseResult {6    const value: number = Number(text);7    if (Number.isNaN(valueNaN)) {8        return { ok: false, error: "not a number" };9    }

  4. parsed ← [object Object]

    13const rawCount: string = "x";14const parsed→ [object Object]: ParseResult = parseCount(rawCountx);

  5. else

    17    console.log(`count=${parsed.value}`);18} else {19    console.log(`error=${parsed.errornot a number}`);20}
    outputerror=not a number

  1. rawCount ← 7

    10    return { ok: true, value: value };11}1213const rawCount→ 7: string = "7";14const parsed: ParseResult = parseCount(rawCount7);

  2. value ← 7

    2    | { ok: true; value: number }3    | { ok: false; error: string };45function parseCount(text7: string): ParseResult {6    const value→ 7: number = Number(text7);7    if (Number.isNaN(value)) {8        return { ok: false, error: "not a number" };9    }10    return { ok: true, value: value7 };11}

  3. parsed ← [object Object]

    13const rawCount: string = "7";14const parsed→ [object Object]: ParseResult = parseCount(rawCount7);

  4. if (parsed.ok)

    13const rawCount: string = "7";14const parsed: ParseResult = parseCount(rawCount);1516if (parsed.oktrue) {17    console.log(`count=${parsed.value7}`);18} else {
    outputcount=7