Mastering Typescript by Nathan Rozentals

Author:Nathan Rozentals
Language: eng
Format: mobi, epub
Tags: COM060160 - COMPUTERS / Web / Web Programming, COM051000 - COMPUTERS / Programming / General, COM060180 - COMPUTERS / Web / Web Services & APIs
Publisher: Packt
Published: 2021-04-22T07:52:48+00:00

Here, we can see that the first value of the stream was processed correctly, but the second value caused the code to crash. This is because the second value emitted in our Observable stream does not have an id property, it is undefined, and we cannot access the value property of an undefined value.

While this may seem a fairly contrived example, it can happen quite often when receiving data in a nested JSON structure. It is always good practice to use optional chaining when dealing with nested properties coming from an outside source, where you can't be sure whether the value has been set correctly.

In order to catch this error correctly, we can provide an error handling function when we subscribe to our Observable stream, as follows:

returnIdValue.subscribe( // called for each observable value (value: number | null) => { console.log(`received ${value} `); }, // called if an error occurs (error: unknown) => { console.log(`error : ${error}`); }, // complete function () => { console.log(`complete`); } );

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