TypeScript: Type guard does not work with switch statement when putting object into variable

Question

I don't understand why type guard doesn't work in the example below...

When we have these interfaces,

interface ParamA {
    name: 'A';
    aaa: boolean;
}

interface ParamB {
    name: 'B';
    bbb: number;
}

Good

function func(param: ParamA | ParamB) {
    switch (param.name) {
        case 'A':
            const aaa = param.aaa;
            console.log(aaa); // boolean
            break;
        case 'B':
            const bbb = param.bbb;
            console.log(bbb); // number
            break;
        default:
            break;
    }
}

Bad

function func(param: ParamA | ParamB) {
    const name = param.name; // just rewrite here
    switch (name) {
        case 'A':
            const aaa = param.aaa;
            //                ^^^
            console.log(aaa);
            break;
        case 'B':
            const bbb = param.bbb;
            //                ^^^
            console.log(bbb);
            break;
        default:
            break;
    }
}

Compiler throws errors like Property 'aaa' does not exist on type 'ParamB'. I do not think there should be a difference in behaviors whether I can put it in a variable.

The version of TypeScript using is 2.8.3.

Can anyone explain this?

Answer 1

switch statement serves as type guard in 'good' snippet and doesn't do that in 'bad' one.

I do not think there should be a difference in behaviors whether I can put it in a variable.

There's no difference in behaviour, there's a difference in how TypeScript is able to analyze this code. Since the code is statically analyzed, the ways how param type can be narrowed are limited.

In order for param to be narrowed down to ParamA or ParamB it should be mentioned in type guard. Since it wasn't but name variable was, param won't be narrowed down, its type will need to be asserted in this case:

...
switch (name) {
    case 'A':
        const aaa = (<ParamA>param).aaa;
...

Answer 2

Update 2021-08:

Typescript 4.4 got smarter and your example work out of the box :-)

Old answer

Assigning the property to a variable looses the connection to the object. This would be very hard to follow for the compiler as you could change the string and the object variable independently:

var param: ParamA | ParamB;
param = someParamA;
const name = param.name;
param = someParamB;
switch (name) { // boom

So if you know for sure that you have not changed the variable you can use the type assertion but i would not do that for maintenance reason.