2

Pseudocode

doStuff() {
  // A is an asynchronous call
  A
  .then(result_a => { // process result_a })
  .then(result_b => { // process result_b })
  .then(() => { // side_effects})
}

How to return result_b?

Background

I'm using TypeORM to save an order and order lines to a sqlite database for an angular+electron desktop application. I don't think this is specific to Angular. I'm not too familiar with using then other than chaining them in a very linear fashion.

Problem Statement

How do I return the purchase_order which is in the middle of the then chain? I'm looking for this or the purchase_order.id.

Stripped down version:

addOrder(header: PurchaseOrder, lines: PurchaseOrderLine[]) {
  const new_order = new PurchaseOrder();

  this.databaseService.connection
    .then(conn => conn.manager.save(new_order))
    .then(purchase_order => {
      let new_lines = lines.map(line => {
        const new_order_line = new PurchaseOrderLine();
        this.databaseService
          .connection
          .then(conn => conn.manager.save(new_order_line));
      })
    })
    .then(() => this.reflectChanges());
}

Fuller version:

addOrder(header: PurchaseOrder, lines: PurchaseOrderLine[]) {
  const new_order = new PurchaseOrder();

  // copy header details to new order
  new_order.purchase_order_date = (<any>header.purchase_order_date).format();
  new_order.total_value = header.total_value;
  if(header.delivery_required_by) {
    new_order.delivery_required_by = (<any>header.delivery_required_by).format();
  }
  new_order.notes = header.notes;
  new_order.supplier = _state.selectedSupplier;

  // use databaseService to save order and then save the order lines
  this.databaseService
    .connection
    .then(conn => conn.manager.save(new_order))
    .then(purchase_order => {
      let new_lines = lines.map(line => {
        const new_order_line = new PurchaseOrderLine();

        new_order_line.quantity = line.quantity
        new_order_line.code = line.code
        new_order_line.file_no = line.file_no
        new_order_line.description = line.description
        new_order_line.unit = line.unit
        new_order_line.rate = line.rate
        new_order_line.total = line.total
        new_order_line.note = line.note
        new_order_line.purchase_order = purchase_order

        this.databaseService
          .connection
          .then(conn => conn.manager.save(new_order_line));
      })
    })
    .then(() => this.reflectChanges());
}
Andrew Allen
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    Just to be clear, you *can't* return the value from the function. You *can* return a promise that resolves to `result_b`. Is that what you mean? I'm asking because your `addOrder()` function doesn't return anything. – Mark Jun 27 '19 at 03:47
  • I'll be honest, I'm struggling to understand what a promise is. Like I know it's the result of async call but once I see code that has `Promise.all` and `then` and `return` all mixed up I know I'm out of my depth. I'd be happy with a `.then(() => return result_b)` at the end of the then chain calls if this is what you mean by returning a promise that resolves to `result_b`? – Andrew Allen Jun 27 '19 at 03:53
  • 1
    Nevermind, thank you @MarkMeyer. Your comment "you can return a promise that resolves to result_b" has lifted me out my confusion. I was today years old when I understood promises :) ```doStuff() { // A is an asynchronous call let promiseA = A.then(result_a => { // process result_a }) let promiseB = promiseA.then(result_b => { // process result_b }) let promiseC = promiseB.then(() => { // side_effects}) return Promise.all([promiseA, promiseB, promiseC]) }``` and I can access `promiseB` by doing `doStuff().then((a, b, c) => b)` – Andrew Allen Jun 27 '19 at 04:40

0 Answers0