add an option that flags coercion

There is a category of bugs caused by unintended coercion at runtime. Such bugs can be found by the compiler. Having an option that bans such coercion would be a great addition.

examples of the problem:

// before (typechecks)
let isFirst: boolean;
// ... later in code
if (!isFirst) {
   // do stuff
}

// after refactoring (still typechecks)
let isFirst: () => boolean;
// ... later in code
if (!isFirst) {
  // got a bug, unreachable code
}

// before (typechecks)
let counter: number;
let kind: string;
// ... later in code
let id = '--' + kind + (counter++); // works: --blah-blah-341279

// after refactoring (still typechecks)
let counter: number;
let kind: {
    type: string;
    sort: string;
};
// ... later in code
let id = '--' + kind + (counter++); // now a bug: --[object Object]-341279

Your first example is identical to a suggestion you've already logged.

Your second example doesn't typecheck.

technically it was about a more specific case (and probably because of that it got closed), which alone didn't make much sense, this is a broader request that might have more sense
fixed

another example out of the oven

// before
declare function toBypassReasons(): string[];
let message = toBypassReasons().join(', ');

// after refactoring
interface BypassReason { reason: string; ruleKey: string; }
declare function toBypassReasons() : BypassReason[];
let message = toBypassReasons().join(', '); // <-- bug

There is literally zero coercion in that example. You'd need completely different mechanics (somehow disallowing certain methods when a type parameter is one value vs another) to detect that "bug" vs the other two examples.

closing in favor of #7989

Tracking a new issue at #9041 that solves some of these cases

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add an option that flags coercion #7746

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add an option that flags coercion #7746

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RyanCavanaugh commented on Mar 31, 2016

zpdDG4gta8XKpMCd commented on Mar 31, 2016

zpdDG4gta8XKpMCd commented on Apr 2, 2016

RyanCavanaugh commented on Apr 2, 2016

mhegazy commented on Jun 7, 2016

RyanCavanaugh commented on Jun 9, 2016

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