Type narrowing for fields

[laszlojakab]

Hi!

Type narrowing is not working (not supported?) for fields (VS2013, TypeScript 1.4).

class O1Cls {
    x: any;
}

class O2Cls {
    y: any;
}

interface Options {
    opt: O1Cls|O2Cls;
}

class MyClass {
    constructor(options: Options) {
        if (options.opt instanceof O1Cls) {
            console.log(options.opt.x); // Error: Property 'x' does not exists on type 'O1Cls | O2Cls'
        }

        var opt = options.opt;
        if (opt instanceof O1Cls)
        {
            console.log(opt.x); // OK
        }
    }
}

Will it be supported or is this a "by design" behaviour?

[lbguilherme]

Fields can have getters, so options.opt may return different things each time it is evaluated. Not sure if it can be statically checked.

[laszlojakab]

@GuilhermeBernal: since i wrote this I've already noticed that. So probably it is not an issue.

[RyanCavanaugh]

See also #1706