Having trouble with JSON parsing in Promise execution

Question

Having trouble with JSON parsing in Promise execution

I am in the process of developing a Promise with the objective of adding up any numbers discovered within an array or JSON object.

The add() function is designed to receive a series of URLs as a string input and calculate the total sum of those URLs.

Here's my approach:

I've introduced a helper function named parse, which accepts a string (the URL) and should display the JSON object specified at that location.

My issue arises when looking at the section marked by $$$$, where the jsonObject correctly represents the parsed JSON object. However, moving on to the area indicated by **** right after calling parse, the parsedObj no longer reflects the accurately parsed JSON object.

For instance, if I try to parse a JSON object that includes [1, 2, 5, 4], I'll receive jsonObject = 1,2,5,4 while parsedObj = [object Promise].

Why does parsedObj show an array of [object Promise] instead of 1,2,5,4?

javascript json parsing typescript

Answer 1

Answer №1

parse(url: string): any will return a Promise instead of your jsonObject

Therefore,

                Log.trace("url: " + s.toString());
                let parsedObj = that.parse(s);
                Log.trace("parsedObj: " + parsedObj); // ****

This process is truly asynchronous. To obtain the jsonObject, you need to do the following:

parsedObj.then(function(jsonObject){
                for (var element of jsonObject) {
                    if (element.isNumber()) {
                        sum += element;
                        countNums++;
                    }
                    Log.trace("sum: " + sum);
                }

                Log.trace("sum: " + sum);
})

You should keep track of your parsedObj (Promises) in an array so that you can use Promise.all() to fulfill your parent promise once all the work is complete

Answer 2

parse(url: string): any will return a Promise instead of your jsonObject

Therefore,

                Log.trace("url: " + s.toString());
                let parsedObj = that.parse(s);
                Log.trace("parsedObj: " + parsedObj); // ****

This process is truly asynchronous. To obtain the jsonObject, you need to do the following:

parsedObj.then(function(jsonObject){
                for (var element of jsonObject) {
                    if (element.isNumber()) {
                        sum += element;
                        countNums++;
                    }
                    Log.trace("sum: " + sum);
                }

                Log.trace("sum: " + sum);
})

You should keep track of your parsedObj (Promises) in an array so that you can use Promise.all() to fulfill your parent promise once all the work is complete

Answer 3

Answer №2

It seems like you may be assuming that parse() will automatically return the resolved value just because there is a Promise.all() in your code.

Your code appears to be quite complex and relies on the deferred antipattern, so I have simplified it for you.

parse(url: string): any {
    let rp = require('request-promise-native');
    return rp({ uri: url })
        .then(
            JSON.parse, 
            function(err){ 
                throw "Error: URL could not be retrieved" 
            }
        );
}

add(urls: string[]): Promise<number> {
    return Promise.all(urls.map(this.parse))
        .then(function(arrays) {
            let emptyArray = [];
             //flatten
            let numbers = emptyArray.concat.apply(emptyArray, arrays)
                //remove non-numeric entries
                .filter(function(v){
                    return v.isNumber();
                });

            if(0 === numbers.length){
                throw "Error: No number was provided";
            }

            //sum
            return numbers.reduce(function(a,b){ 
                return a+b;
            });
        });
}

However, I have omitted the logs from the code.

Do you have any questions?

Answer 4

It seems like you may be assuming that parse() will automatically return the resolved value just because there is a Promise.all() in your code.

Your code appears to be quite complex and relies on the deferred antipattern, so I have simplified it for you.

parse(url: string): any {
    let rp = require('request-promise-native');
    return rp({ uri: url })
        .then(
            JSON.parse, 
            function(err){ 
                throw "Error: URL could not be retrieved" 
            }
        );
}

add(urls: string[]): Promise<number> {
    return Promise.all(urls.map(this.parse))
        .then(function(arrays) {
            let emptyArray = [];
             //flatten
            let numbers = emptyArray.concat.apply(emptyArray, arrays)
                //remove non-numeric entries
                .filter(function(v){
                    return v.isNumber();
                });

            if(0 === numbers.length){
                throw "Error: No number was provided";
            }

            //sum
            return numbers.reduce(function(a,b){ 
                return a+b;
            });
        });
}

However, I have omitted the logs from the code.

Do you have any questions?

Having trouble with JSON parsing in Promise execution

Answer №1

Answer №2

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