Allow switch type guards

[icholy]

I have the following code:

function logNumber(v: number) { console.log("number:", v); }
function logString(v: string) { console.log("string:", v); }

function foo1(v: number|string) {
    switch (typeof v) {
        case 'number':
            logNumber(v);
            break;
        case 'string':
            logString(v);
            break;
        default:
            throw new Error("unsupported type");
    }
}

Error:

Argument of type 'string | number' is not assignable to parameter of type 'number'.
 Type 'string' is not assignable to type 'number'.

I was forced to rewrite this using if statements.

function foo2(v: number|string) {
    if (typeof v === 'number') {
        logNumber(v);
    } else if (typeof v === 'string') {
        logString(v);
    } else {
        throw new Error("unsupported type");
    }
}

Please allow using switch statements as type guards.

Yes please allow switch case there for state-machine. Also the conditional/ternary operator (?:).

[zspitz]

+1 for switch and ?:
@RyanCavanaugh @jasonwilliams200OK Isn't this the same as #2388 ?
@icholy In the meantime, you could use explicit type casting:

switch (typeof v) {
    case 'number':
        logNumber(<number>v);
        break;
    case 'string':
        logString(<string>v);
        break;
    default:
        throw new Error("unsupported type");
}```

[RyanCavanaugh]

Approved

[AbraaoAlves]

+1

[jednano]

Is this the same as #2388?

[DanielRosenwasser]

Not quite the same thing, since you don't need #2388 for this.