Enum with arguments typescript

Question

Swift can do this:

enum Barcode {
    case upc(Int, Int, Int, Int)
    case qrCode(String)
}

Is there a way to include arguments in enum cases like that in Typescript? Thanks.

What Swift and Java refer to as an enum, other languages (including TypeScript) refer to as "discriminated-unions". — Dai
– Dai, Commented Oct 16, 2021 at 23:19
Which UPC specification are you using, btw? There isn't necessarily 4 modules in a UPC, so you should be specific about which spec you're using (e.g. UPC-A) — Dai
– Dai, Commented Oct 16, 2021 at 23:22
Ahhh yep that's what im looking for. Haha not doing anything with barcodes just found that example online. Thanks though!! — JJT
– JJT, Commented Oct 16, 2021 at 23:24

Dai · Accepted Answer · 2021-10-16 23:30:20Z

What Swift and Java refer to as an enum, other languages (including TypeScript) refer to as "union type".

https://www.typescriptlang.org/docs/handbook/unions-and-intersections.html

TypeScript offers many ways to skin a cat, but one idiomatic implementation in TypeScript equivalent is:

type Upc     = { S: number, L: number, M: number, R: number, E: number };
type QRCode  = { asText: string };
type Barcode = Upc | QRCode;

As TypeScript uses type-erasure there isn't any runtime information that immediately self-describes a Barcode object as either a Upc or a QRCode, so in order to discriminate between Upc and QRCode you'll need to write a type-guard function too:

function isUpc( obj: Barcode ): obj is Upc {
    const whatIf = obj as Upc;
    return (
        typeof whatIf.S === 'number' &&
        typeof whatIf.L === 'number' &&
        typeof whatIf.M === 'number' &&
        typeof whatIf.R === 'number' &&
        typeof whatIf.E === 'number'
    );
}

function isQRCode( obj: Barcode ): obj is QRCode {
    const whatIf = obj as QRCode;
    return typeof whatIf.asText === 'string';
}

Used like so:

async function participateInGlobalTrade() {
    
    const barcode: Barcode = await readFromBarcodeScanner();
    if( isUpc( barcode ) ) {
        console.log( "UPC: " + barcode.S ); // <-- Using `.S` is okay here because TypeScript *knows* `barcode` is a `Upc` object.
    }
    else if( isQRCode( barcode ) ) {
        console.log( "QR Code: " + barcode.asText ); // <-- Using `.asText` is okay here because TypeScript *knows* `barcode` is a `QRCode` object.
    }
}

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