danger.ts

// Goal: make mistakes like the first function fail to compile.

import * as $ from 'jquery';

function makeThisNotCompile(): number {
    // NOTE: With @types/jquery,
    //   this is defined to return 'any' (and very likely, NOT a number).
    // The typechecker has been rendered useless, yet this looks no different
    //  at all from any piece of code where the type checker CAN be trusted.
    //
    // --noImplicitAny does not catch this because it does not apply.
    return $("input").val();
}

function allowThisToCompile(): number {
    // With an explicit type assertion, the potential for danger becomes clear.
    return $("input").val() as number;
}