How can we use the AsyncIterator properly

Question

I'm trying to define an AsycIterator and use it. Defining works but using it has been a problem. This is the code I have, please note that other code not related to the question has been removed

This is how I define the iterator

export interface IDriver {

  tables(): AsyncIterator<Table>
  
 // code removed
}

This is how I implement it.

export class MySql implements IDriver {
  async *tables(): AsyncIterator<Table> {
    const sql = `select table_name, table_type, table_rows 
                 from information_schema.tables 
                 where table_schema = ${this.schema} 
                 order by table_name;`

    const rows = await this.query<{
      table_name: string
      table_type: string
      table_rows: number
    }>(sql)

    for (const row of rows) {
      yield {
        name: row.table_name,
        rows: row.table_rows,
        type: row.table_type
      } as Table
    }
  }

 // ---
}

This is my package.json file

{
  "compilerOptions": {
    "target": "ESNext" ,
    "lib": [
      "ES2018.AsyncIterable",
      "DOM",
      "ESNext"
    ] ,
    "jsx": "preserve" ,
    "experimentalDecorators": true ,
    "emitDecoratorMetadata": true ,
    "useDefineForClassFields": true ,
    "moduleDetection": "auto" ,
    
    "module": "NodeNext", 
    "rootDir": "src/" ,
    "moduleResolution": "NodeNext" ,
  
    "allowUmdGlobalAccess": true ,
    "resolvePackageJsonExports": true ,
    "resolvePackageJsonImports": true ,
    "resolveJsonModule": true ,
    "checkJs": true ,
     
    "declaration": true ,
    "declarationMap": true ,
    "sourceMap": true ,
     "outDir": "./build" ,
    "removeComments": true ,
   
    "newLine": "crlf" ,
    "stripInternal": true ,
    "preserveConstEnums": true ,
      
    "allowSyntheticDefaultImports": true ,
    "esModuleInterop": true ,
    "forceConsistentCasingInFileNames": true ,

    "strict": true ,
    "noImplicitAny": true ,
    "strictNullChecks": true ,
    "strictFunctionTypes": true ,
    "strictBindCallApply": true ,
    "strictPropertyInitialization": true ,
    "noImplicitThis": true ,
    "useUnknownInCatchVariables": true ,
    "alwaysStrict": true ,
    "noUnusedLocals": true ,
    "noUnusedParameters": true ,
    "exactOptionalPropertyTypes": true ,
    "noImplicitReturns": true ,
    "noFallthroughCasesInSwitch": true ,
    "noUncheckedIndexedAccess": true ,
    "noImplicitOverride": true ,
    "noPropertyAccessFromIndexSignature": true ,
    "skipLibCheck": true ,
  },
  "include": ["./src/*"]
}

This is the usage of the above function

for await (const it of mysql.tables()) {
    print(it)
  }

which gives me a an error

src/main.ts:13:26 - error TS2504: Type 'AsyncIterator<Table, any, undefined>' must have a [Symbol.asyncIterator] () method that returns an async iterator.

If I try to explore it, this is what I find in type definitions:

interface AsyncIterable<T> {
    [Symbol.asyncIterator](): AsyncIterator<T>;
}
interface SymbolConstructor {
    /**
     * A method that returns the default async iterator for an object. Called by the semantics of
     * the for-await-of statement.
     */
    readonly asyncIterator: unique symbol;
}
declare var Symbol: SymbolConstructor;

I also tried this solution given here:

How do async iterators work? error TS2504: Type must have a '[Symbol.asyncIterator]()' method that returns an async iterator

It is a 6 year old question as of now and does not work at all in my case. core-js/shim has no effect. I don't know if the solution was only valid for a particular version of typecript. I'm using Version 5.5.4. the only way I'm able to use AsyncIterator is given below.

  const rows = await mysql.tables()
  let item = await rows.next()
  while (!item.done) {

    item = await rows.next()
  }

Which obviously is a weird way to use AsyncIterator, Really if this is the solution then perhaps not using AsyncIterator is the best option

Does anyone know what changes are required to make the code for await () work or Do i need to change my code?

Please note that I cannot return an array of my dataset because line by line processing is really needed.

Diego D · Accepted Answer · 2024-09-11 08:56:57Z

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of

The for await...of statement creates a loop iterating over async iterable objects as well as sync iterables. This statement can only be used in contexts where await can be used, which includes inside an async function body and in a module.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_async_iterator_and_async_iterable_protocols

In order to be iterable, an object must implement the Symbol.iterator method, meaning that the object (or one of the objects up its prototype chain) must have a property with a [Symbol.iterator] key which is available via constant Symbol.iterator:

Symbol.iterator A zero-argument function that returns an object, conforming to the iterator protocol.

Whenever an object needs to be iterated (such as at the beginning of a for...of loop), its Symbol.iterator method is called with no arguments, and the returned iterator is used to obtain the values to be iterated.

https://tc39.es/ecma262/#sec-asynciterable-interface

%Symbol.asyncIterator% - a function that returns an AsyncIterator object - The returned object must conform to the AsyncIterator interface.

To make it short, to iterate over mysql.tables() like this...

for await (const it of mysql.tables()) {/*...*/}

You .tables() must return an iterable object, thus to make your code working you need to use AsyncIterable instead of AsyncIterator:

interface IDriver {
  tables(): AsyncIterable<Table>;
}

/*...*/

async *tables(): AsyncIterable<Table> {/*...*/}

TS Playground DEMO

interface Table {
  name: string;
  rows: number;
  type: string;
}

interface IDriver {
  tables(): AsyncIterable<Table>;
}

class MySql implements IDriver {
  
  private schema: string = 'myschema';    
  private async query<T>(sql: string): Promise<T[]> {        
    return [
      { table_name: 'table1', table_type: '***', table_rows: 10 },
      { table_name: 'table2', table_type: '***', table_rows: 20 },
    ] as unknown as T[];
  }  
 
  async *tables(): AsyncIterable<Table> {
    const sql = `select table_name, table_type, table_rows 
                 from information_schema.tables 
                 where table_schema = '${this.schema}' 
                 order by table_name;`;

    const rows = await this.query<{
      table_name: string;
      table_type: string;
      table_rows: number;
    }>(sql);
    
    for (const row of rows) {
      yield {
        name: row.table_name,
        rows: row.table_rows,
        type: row.table_type,
      } as Table;
    }
  }
}

async function run() {
  const mysql = new MySql();
  
  for await (const table of mysql.tables()) {
    console.log(table);
  }
}

i didn't really knew the diffeence between AsyncIterable and AsyncIterator. thanks a lot
if you could verify yourself that the solution is working in your scenario, feel free to accept the answer given

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