Type inference doesn't discard 'undefined' possibility of an optional property inside an if(obj[name].optionalProperty)

[lmcarreiro]

TypeScript Version: 2.6.0-dev.20171015

Code

interface A {
    arrayOfB?: B[];
}

interface B {
    prop: number;
}

var obj: { [name: string]: A } = {};

for (let name in obj) {
    if (obj[name].arrayOfB) { // make sure that obj[name].arrayOfB is not undefined
        for (let b in obj[name].arrayOfB) { // error in 'obj[name].arrayOfB': [ts] Object is possibly 'undefined'.

        }
    }

    // in this way it is ok, no errors
    let arrayOfB = obj[name].arrayOfB || [];
    for (let b in arrayOfB) {
        
    }
}

Expected behavior:
Infer that obj[name].arrayOfB is of type B[]

Actual behavior:
Considering obj[name].arrayOfB of type B[]|undefined

[fenying]

Yes, I'm in trouble of this problem, that really make me sick....

I wish this could be solved ASAP. T_T

[lmcarreiro]

You could use this pattern, I use it for string.match(/regex/) too:

    let arrayOfB = obj[name].arrayOfB || [];
    for (let b in arrayOfB) {
        
    }

    let matches = ("string".match(/regex/) || []);
    matches.forEach(/*...*/);

[RyanCavanaugh]

#56389