type guards strangeness

[zpdDG4gta8XKpMCd]

TypeScript Version:

1.8.0

Code

const enum AsNonBlank {}
export function isNonBlank(value: string) : value is string & AsNonBlank {
 return !/^\s*$/g.test(value);
}
let value: string;
if (isNonBlank(value)) {
 doThis(value); // expected value to be string & AsNonBlank, actual: string
} else {
 doThat(value);
}
function doThis(value: string & AsNonBlank): void {
}
function doThat(value: string) : void {
}

Expected behavior:
value should either be string & AsNonBlank
or there must be a message saying that the type guard is invalid or something
Actual behavior:
value is a string

[yortus]

Aleksey-Bykov I'm also finding that user-defined type guards often don't work the way I intuitively expect. Your example makes intuitive sense to me and I would have expected it to work.

However, since you haven't started with a union type, I suspect you are working outside the bounds of what tsc can currently do with type guards. I got your example to work by changing the declaration of value to:

let value: string | (string & AsNonBlank);

This seems to satisfy the compiler, and the narrowing happens as expected in both the if and else branches.

[yortus]

Interestingly, the following version of your code does work as you expect:

const enum AsUnaryFunc {}
export function isUnaryFunc(value: Function) : value is Function & AsUnaryFunc {
 return value.length === 1;
}
let value: Function;
if (isUnaryFunc(value)) {
 doThis(value); // expected value to be Function & AsUnaryFunc, AND IT IS
} else {
 doThat(value);
}
function doThis(value: Function & AsUnaryFunc): void {
}
function doThat(value: Function) : void {
}

It's the same code as yours, just with Function/AsUnaryFunction instead of string/AsNonBlank.

[zpdDG4gta8XKpMCd]

Troy Gerwien (@yortus) thanks!

[zpdDG4gta8XKpMCd]

Troy Gerwien (@yortus) just checked with the union, it doesn't work either

const enum AsNonBlank {}
export function isNonBlank(value: string | string & AsNonBlank) : value is string & AsNonBlank {
 return !/^\s*$/g.test(value);
}
let value: string;
if (isNonBlank(value)) {
 doThis(value); // expected value to be string & AsNonBlank, actual is *STILL* a string
} else {
 doThat(value);
}
function doThis(value: string & AsNonBlank): void {
}
function doThat(value: string) : void {
}

[yortus]

Sorry I wasn't clear - this one works for me:

const enum AsNonBlank {}
export function isNonBlank(value: string) : value is string & AsNonBlank {
 return !/^\s*$/g.test(value);
}
let value: string | (string & AsNonBlank); // <-- union here
if (isNonBlank(value)) {
 doThis(value); // expected value to be string & AsNonBlank
} else {
 doThat(value);
}
function doThis(value: string & AsNonBlank): void {
}
function doThat(value: string) : void {
}

[zpdDG4gta8XKpMCd]

Thanks! That's a great help!