Observables and the categorization of response data

Understanding Observables can be a bit tricky for me at times, leading to some confusion.

Let's say we are subscribing to getData in order to retrieve JSON data asynchronously:

this.getData(id)
    .subscribe(res => {
        console.log(data.items[0])
        // more data processing
    })

This approach works, but it doesn't seem very elegant to process the response data within the .subscribe block. Storing the response in a variable seems like a better option:

let data;
this.getData(id)
        .subscribe(res => data = res)
console.log(data.items[0])
// more data processing

However, when trying this method, an error occurs due to 'data' not having an initial type.

TypeError: Cannot read property 'items' of undefined

Creating an interface for the JSON response might seem unnecessary. What am I missing?

Furthermore, setting up a callback function appears redundant as it requires using two functions to achieve what should ideally be done with just one.

Answer №1

since variable 'information' does not have an initial data type.

No, the issue arises because data is actually undefined. TypeScript is intelligent enough to detect that data has not been assigned any value yet. The sequence of execution for the code is as follows:

let data; // 1 
this.getData(id)
        .subscribe(res => data = res) // 3!
console.log(data.items[0]) // 2

Further Details

I recommend researching async programming in JavaScript / TypeScript. Simply put, you can only access the data once the subscribe method is triggered.

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