Infer type guards from `instanceof` and other such expressions

[chuckjaz]

Consider the following from here:

const asAndBs = [new A(), new B(), new A(), new B()];
const allAs = asAndBs.filter(v => v instanceof A); 

The type of allAs is (A | B)[] where it would be more accurately typed as A[]. If a type-guard was used,

function isA(v: any): v is A {
    return v instanceof A;    
}

const allAs = asAndBs.filter(isA);

then the type would be A[] as expected.

Getting the correct type from filter requires using the verbose form filter(isA) instead of the more terse form filter(v => v instanceof A).

Suggestion

Infer a guard from lambdas that imply a type narrowing if used in an if statement.

For example,

const a = (v: any) => v instanceof Class;

would infer a to be of type (v: any) => v is Class.

const b = (v: any) => typeof v === 'string'

would infer b to be type (v: any) => v is string;

This would allow useful typing of:

const loweredStrings = someArray.filter(v => typeof v === 'string').map(v => v.toLower());

[jcalz]

One of many duplicates of #5101. Did you try to search for related issues before posting this?

[chuckjaz]

I tried. My search foo is clearly lacking.

[jcalz]

Sure. I only ask because the issue posting template asks you to list what search terms you used... well, I guess it says that specifically for bug reports as opposed to suggestions.

I have this fantasy that one day we will find the magic wording that eliminates duplicate issues while still encouraging legitimate bug reports and suggestions. But sadly it is only a dream.

[aluanhaddad]

@jcalz sophisticated lexical analysis should be employed by GitHub searches :)

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.