Unfortunately it doesn't work in TS as well,
if you run this in TS playground (I guess it has the latest TS version)

let obj = {};
Object.defineProperty(obj, "x", { value: "hello", writable: false });

obj.x.toLowercase();

then it will show only one error message:
"Property 'x' does not exist on type '{}'"
but I expected to get different error like "Property 'toLowercase' does not exist on type 'string'" as was described in https://devblogs.microsoft.com/typescript/announcing-typescript-3-2/#objectdefineproperty-declarations-in-javascript

@DanielRosenwasser See #14650

这是关于JavaScript分析的，而不是TypeScript。

我遇到了这个问题。有人知道解决方法吗？

n/m,上述问题的解决方法如下：

// @ts-check

class C {
  constructor() {
    /** @type {Readonly<string>} */
    this.x = undefined;
    Object.defineProperty(this, "x", { value: "hello" });

    this.x.toLowerCase();
  }
}

你需要为属性 'x: string' 添加确定性Assert赋值。

我认为在这个问题中，源代码被假定为JavaScript,而且据我所知，没有明确的赋值Assert语法。