delete operator and strict null checks

[jmlopez-rod]

TypeScript Version: 2.3 (the one in the playground)

Code

interface IName {
    name?: string;
}

class A {
    prop: IName;

    constructor(param: IName) {
        this.prop = param;
    }

    printProp() {
        if (this.prop) {
            // this.prop = undefined;  // TS will catch this error
            delete this.prop;  // Oh no, now we'll have a runtime error
            console.log(this.prop.name);
        }
    }
}

const a = new A({ name: 'a' });
a.printProp();

Or click here to see it in the playground.

Expected behavior:

Should warn us about using the delete operator since we told typescript that prop is an IName.

Actual behavior:

Currently as the code stands typescript is not able to warn us of the oncoming failure. I have left the line this.prop = undefined and typescript will complain about it. Using the delete operator will do the same as setting as undefined but there is no warning.

Make sure to use the strictNullChecks option when running the code.

[mhegazy]

looks like a duplicate of #13783

[jmlopez-rod]

That seems to be the case.