TypeChecker::getTypeAtLocation(Identifier) does not Return Narrowed Type Inside an Array.isArray()-Based Type Guard

[SlurpTheo]

1.8.10 / next (1.9.0-dev.20160414)

Code

export function X() {
    let x: number | number[] = 0;

    if (typeof x === "number") {
        x;              // ts.createProgram(...).getTypeChecker().getTypeAtLocation(Identifier::x).getFlags() --> number
        const y = x;    // ts.createProgram(...).getTypeChecker().getTypeAtLocation(Identifier::y).getFlags() --> number
    }
    if (typeof x !== "object") {
        x;              // ts.createProgram(...).getTypeChecker().getTypeAtLocation(Identifier::x).getFlags() --> number
        const y = x;    // ts.createProgram(...).getTypeChecker().getTypeAtLocation(Identifier::y).getFlags() --> number
    }
    if (!Array.isArray(x)) {
        x;              // ts.createProgram(...).getTypeChecker().getTypeAtLocation(Identifier::x).getFlags() --> Union
        const y = x;    // ts.createProgram(...).getTypeChecker().getTypeAtLocation(Identifier::y).getFlags() --> number
    }

Behavior:
Why does TypeChecker::getTypeAtLocation(Identifier::x) correctly report number after the first (typeof x === "number") & second (typeof x !== "object") type guards, but report the (un-narrowed) Union type after the third (!Array.isArray(x)) type guard? All three block's const c is correctly typed as number. Is there something besides TypeChecker::getTypeAtLocation(Identifier) to use here?

[mhegazy]

how about getSymbolAtLocation(), getTypeOfSymbolAtLocation(symbol, node) ? do you see any difference?

[SlurpTheo]

Doing getTypeOfSymbolAtLocation(getSymbolAtLocation(Identifier::x), Identifier::x) gives me the same (Union) type, yes (that also correctly reports number in the first two type guard blocks).

[SlurpTheo]

This appears fixed in typescript@next (1.9.0-dev.20160426) -- probably by #8010

... although, in this version of the code if you add an else to !Array.isArray(x):

export function X() {
    let x: number | number[] = 0;

    if (!Array.isArray(x)) {
        const y = x;
    } else {
        const z = x;
    }

You end up with the type of variable z being ts.TypeFlags.Anonymous...

[mhegazy]

i believe this should be addressed by #8324.

[SlurpTheo]

OK, tried typescript@next (1.9.0-dev.20160428) and now I get number reported in both branches for TypeChecker::getTypeAtLocation(Identifier::x), but TypeChecker::getTypeAtLocation(VariableDeclaration::z) is returning ts.TypeFlags.Anonymous.

[mhegazy]

this is control flow analysis in action :D

let x: number | number[] = 0;

// x here can only be a number, you assigned 0 to it

if (!Array.isArray(x)) {
   // it is a number not an array alright
   const y = x;
} else {
   // it was a number, now we can only be in this branch if it is an array, but it ain't. so it is nothing.
    const z = x;
}

this should do what you expect:

let x: number | number[]; // notice, no initialization

if (!Array.isArray(x)) {
    const y = x; // number
} else {
    const z = x; // number[]
}

[mhegazy]

looks like this was fixed. closing.

[SlurpTheo]

Mohamed Hegazy (@mhegazy) That... is... holy!!!

I think I'm in love with this 👍