Iterables 表现不佳

Question

I have a bizarro issue which I just ran into... when passed an array of objects, the for..of iterator has no problem iterating over the objects and giving me the "name" property of each. However, array.forEach completely fails to iterate.

The main function is found in Database.ts :

import { ITableDefinition } from "./Table";

export function Database(...tables: ITableDefinition<any>[]) {
  const names: string[] = [];
  for (const table of tables) {
    names.push(table.name);
  }

  return {
    tables,
    tableNames: tables.forEach((table) => table.name),
    tableNamesWithForOf: names,
  };
}

As you can see the function reflexively passes the array of tables it receives but also passes the name property of each table and it does it twice . Once using forEach and once using for..of .

When running the following Jest tests, 2 of the 3 pass but the forEach test fails:

describe("Trouble with iterables => ", () => {
  // db.tables is received as an array
  it("tables property is an array", () => {
    const db = Database(Table(Song), Table(Playlist));
    expect(Array.isArray(db.tables)).toBe(true);
    expect(typeof db.tables[0]).toBe("object");
  });

  // for..of iterates over the array correctly and extracts the "name" prop
  it("for ... of returns the names of the tables correctly", () => {
    const db = Database(Table(Song), Table(Playlist));

    expect(db.tableNamesWithForOf).toHaveLength(2);
    expect(db.tableNamesWithForOf).toContain("Song");
    expect(db.tableNamesWithForOf).toContain("Playlist");
  });

  // shockingly, forEach doesn't iterate at all and returns `undefined`!
  it("forEach returns the names of the tables correctly", () => {
    const db = Database(Table(Song), Table(Playlist));

    expect(db.tableNames).toHaveLength(2);
    expect(db.tableNames).toContain("Song");
    expect(db.tableNames).toContain("Playlist");
  });
});

My mind is blown but I'd love to hear from you if you can make some sense of it.

• Git Repo - all code and tests
• GitPod Container ... new to GitPod, hopefully the link works (super convenient)

Note: I tagged as both javascript and typescript but i doubt this has to with transpiling so if folks think I should take off the TS tag let me know and I'll do so.

Answer 1

forEach in tables.forEach((table) => table.name) doesn't return anything i.e. returns undefined , you can use .map instead