Support asserting !isUndefined/!isNullOrUndefined

[mqudsi]

TypeScript Version: 2.9

Search Terms: assert, isUndefined, strictNullChecks, assertion

Code

function assert(foo: boolean, message: string = "") {
    if (!foo) {
        console.error("Assertion failure:", message);
        throw "Assertion failure: " + message;
    }
}

class Bar {
    foo?: number;

    baz(): void {
        assert(this.foo != undefined); //or assert(!isUndefined(this.foo))
        this.foo += 7;
    }
}

Expected behavior:
tsc should support this declarative form of coding, and ship with support for assert(expr) which satisfies the compiler's requirement of !!expr. Obviously support for user-defined assertions would be extremely complicated as it would require tsc's analysis to leave the scope it is in, but an in-built assert function would work around that.

Actual behavior:
An error about possibly undefined foo is thrown with strictNullChecks on, pointing to the line this.foo += 7;.

Playground Link:

https://www.typescriptlang.org/play/#src=function%20assert(foo%3A%20boolean%2C%20message%3A%20string%20%3D%20%22%22)%20%7B%0D%0A%20%20%20%20if%20(!foo)%20%7B%0D%0A%20%20%20%20%20%20%20%20console.error(%22Assertion%20failure%3A%22%2C%20message)%3B%0D%0A%20%20%20%20%20%20%20%20throw%20%22Assertion%20failure%3A%20%22%20%2B%20message%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0Aclass%20Bar%20%7B%0D%0A%20%20%20%20foo%3F%3A%20number%3B%0D%0A%0D%0A%20%20%20%20baz()%3A%20void%20%7B%0D%0A%20%20%20%20%20%20%20%20assert(this.foo%20!%3D%20undefined)%3B%20%2F%2For%20assert(!isUndefined(foo))%0D%0A%20%20%20%20%20%20%20%20this.foo%20%2B%3D%207%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A

Related Issues:

Duplicate of #8655

[mqudsi]

Thanks, Andy. I didn't catch it with the keywords I tried earlier.