Notes on “Tour of Heroes” Tutorial: R and U of CRUD

First up in the server interaction portion of this Angular tutorial is R(ead) of CRUD, and we see how to retrieve data using RxJS as an intermediary for making HTTP GET calls. There were unfortunately a few pieces of unexplained magic, such as the presence of a method getHeroNo404() that was explicitly called out in a sidebar yet whose purpose is, strangely, not explained in that sidebar. We can infer it has something to do with error handling, since HTTP 404 is a common error and the tutorial started covering error handling. A very important part of writing any web pap, since HTTP fails all the time for factors out of our control. Still, it would have been nice to know more about how getHeroNo404() fits into the picture.

Next up is U(pdate) of CRUD, and while I’m happy to see some compile-time verification, there’s still some sadness that a lot are still left up to runtime. When I made a mistake in binding button (click)="save()" without defining a save() function in TS, it did not generate a compile time error even though at first glance all the pieces to detect this at compile time are present. In reality, it’s not until runtime and a click on the button do we get the error. And again we need to learn to interpret the error. Because while the lack of save() is correct, the user never declared anything named ctx_r3 and so we had to work backwards by guessing what Angular framework had created on our behalf.

ERROR TypeError: ctx_r3.save is not a function
at HeroDetailComponent_div_0_Template_button_click_12_listener (hero-detail.component.html:10)
at executeListenerWithErrorHandling (core.js:14317)
at wrapListenerIn_markDirtyAndPreventDefault (core.js:14352) at HTMLButtonElement.<anonymous> (platform-browser.js:582)
at ZoneDelegate.invokeTask (zone-evergreen.js:399)
at Object.onInvokeTask (core.js:27428)
at ZoneDelegate.invokeTask (zone-evergreen.js:398)
at Zone.runTask (zone-evergreen.js:167)
at ZoneTask.invokeTask [as invoke] (zone-evergreen.js:480)
at invokeTask (zone-evergreen.js:1621) defaultErrorLogger @ core.js:4197 handleError @ core.js:4245 handleError @ core.js:8627 executeListenerWithErrorHandling @ core.js:14320 wrapListenerIn_markDirtyAndPreventDefault @ core.js:14352 (anonymous) @ platform-browser.js:582 invokeTask @ zone-evergreen.js:399 onInvokeTask @ core.js:27428 invokeTask @ zone-evergreen.js:398 runTask @ zone-evergreen.js:167 invokeTask @ zone-evergreen.js:480 invokeTask @ zone-evergreen.js:1621 globalZoneAwareCallback @ zone-evergreen.js:1647

Thankfully this was towards the end of the tutorial and chances are good anyone following along would have seen enough to know how to diagnose these problems. Because it certainly doesn’t get any easier as the tutorial continues to the remainder of CRUD.

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