如何在打字稿中键入诺言的回调？

Question

I can type the callback for .then in my promise, currently I am temporary using any bit I would like to use string or similar.

Any idea how to type my function md5?

export const md5 = (path: string) =>
  new Promise((resolve, reject) => {
    const hash = createHash("sha1");
    const rs = createReadStream(path);
    rs.on("error", reject);
    rs.on("data", chunk => hash.update(chunk));
    rs.on("end", () => resolve(hash.digest("hex")));
  });


  it("should hash md5 a file", () => {
    // error here on hash:string, if I use instead any it works
    const m = md5(fileName).then((hash: string) => {
      assert.strictEqual(m, "4738e449ab0ae7c25505aab6e88750da");
    });

Error I receive:

Argument of type '(hash: string) => void' is not assignable to parameter of type '(value: {}) => void | PromiseLike<void>'.
  Types of parameters 'hash' and 'value' are incompatible.
    Type '{}' is not assignable to type 'string'.
  });

Answer 1

Consider specifying the return Promise type directly new Promise<string> :

export const md5 = (path: string) =>
  new Promise<string>((resolve, reject) => {
    const hash = createHash("sha1");
    const rs = createReadStream(path);
    rs.on("error", reject);
    rs.on("data", chunk => hash.update(chunk));
    rs.on("end", () => resolve(hash.digest("hex")));
  });

Please also check for an error in your test, replace m with hash :

assert.strictEqual(hash, "4738e449ab0ae7c25505aab6e88750da");

Answer 2

Either give the Promise a generic type parameter, give your md5 method a return type, or both:

export const md5 = (path: string): Promise<string> =>
  new Promise<string>((resolve, reject) => {
    const hash = createHash("sha1");
    const rs = createReadStream(path);
    rs.on("error", reject);
    rs.on("data", chunk => hash.update(chunk));
    rs.on("end", () => resolve(hash.digest("hex")));
  });

Answer 3

All you are missing is 'return'. Change to this:

export const md5 = (path: string) =>
    return new Promise((resolve, reject) => {
        const hash = createHash("sha1");
        const rs = createReadStream(path);
        rs.on("error", reject);
        rs.on("data", chunk => hash.update(chunk));
        rs.on("end", () => resolve(hash.digest("hex")));
    });