Switching between PascalCase and camelCase in TypeScript leads to unexpected behavior

Question

Switching between PascalCase and camelCase in TypeScript leads to unexpected behavior

Currently, I am in the process of transitioning a C# desktop application to an Angular/TypeScript web application.

In the C# application, all class properties are named using PascalCase. Therefore, I decided to maintain this naming convention in the TypeScript classes as well.

Below are examples of two similar TypeScript classes, with one using PascalCase and the other camelCase:

//PascalCase
export class Info 
{
    public ManagedType:string;

    public ApiTemplate:string;
}

//camelCase
export class Info 
{
    public managedType:string;

    public apiTemplate:string;
}

Now, let's discuss the peculiar behavior that I have observed:

When retrieving JSON data from the server and populating an array of the Info class, the naming convention (PascalCase or camelCase) used in the TypeScript class does not affect the data retrieval process.
```
this.Infos = await this.HttpClient.get<Info[]>(this.Url).toPromise<Info[]>();
```
However, upon logging the array to the console, I noticed that the output consistently uses camelCase for the properties, regardless of whether the Info class uses PascalCase or camelCase.
Here is where things get interesting: When attempting to filter the array to retrieve a specific instance of the Info class using PascalCase, the result is always undefined/null.

On the contrary, when filtering the array using camelCase, the specific instance of the Info class is successfully found and returned.

//This approach results in 'Info' being undefined, even though the array exists.
let Info = Infos.filter(i => i.ManagedType == "Something" && i.ApiTemplate == "Something else")[0];

//In contrast, this method correctly retrieves 'Info'.
let Info = Infos.filter(i => i.managedType == "Something" && i.apiTemplate == "Something else")[0];

My queries are:

Why does this discrepancy exist? Is it a TypeScript limitation or an issue specific to Angular?

Is there an implicit convention that I need to adhere to?

And why doesn't the TypeScript compiler warn or throw an error regarding the potential inconsistencies when using PascalCase?

arrays angular typescript camelcasing pascalcasing

Answer 1

Answer №1

What is the root cause of this problem? Is it related to TypeScript or Angular?

It's neither. The issue stems from the fact that the JSON data received from your server does not match the structure and format defined in the Info class in TypeScript.

Is there a specific convention I should adhere to?

Yes, there is. It's important to manually test and ensure that the data structures align with the classes you are casting them to. Before casting a JSON object to a specific class, parse it as a generic object and verify if it includes all the required properties matching the class structure (Info). Only then proceed with the casting process.

UPDATE: To address this issue, consider utilizing User-defined Typeguard functions in TypeScript. These functions can help determine whether an object belongs to a particular type, providing stronger assurance and type guarding capabilities. More information can be found here.

// User-defined type-guard function
function isInfo(obj: Object): obj is Info {
    if ('ManagedType' in obj && 'ApiTemplate' in obj) {
        return true;
    } else {
        return false;
    }
}

// Assume jsonString contains JSON data from an HTTP response body
let obj = JSON.parse(jsonString);

if (isInfo(obj)) {
    obj.ApiTemplate; // TypeScript recognizes obj as type Info in this scope
} else {
    // TypeScript knows obj is NOT of type Info in this scope
}

Why doesn't TypeScript flag the use of PascalCase as a potential issue?

TypeScript doesn't raise an error or warning because when using implicit cast like

this.HttpClient.get<Info[]>(this.Url).toPromise<Info[]>();

, you're essentially telling TypeScript that the returned JSON string will perfectly match the expected Info[] structure. However, due to case sensitivity differences, the actual runtime data may not align with the specified class definition. TypeScript relies on your implicit knowledge for such scenarios.

In essence:

You're converting a JSON object at runtime that may not entirely match the defined Info class due to property name casing discrepancies. This example highlights the issue:

//PascalCase
class Info 
{
    public ManagedType:string;

    public ApiTemplate:string;
}

let jsonString = `{
    "managedType": "1234asdf",
    "apiTemplate": "asdf1234"
}`;

// The following implicitly casts the JSON data to Info object assuming exact compatibility.
let obj: Info = JSON.parse(jsonString);

The above scenario mirrors the blind casting performed by

this.Infos = await this.HttpClient.get<Info[]>(this.Url).toPromise<Info[]>();

By utilizing PascalCase naming in the class definition but receiving camelCase properties in the JSON data, TypeScript cannot foresee these discrepancies during compilation and assumes your runtime conversions will align.

//Typescript expects PascalCase properties but they are actually camelCase - a discrepancy occurs at runtime.
console.log(`accessing Info.ManagedType property: ${obj.ManagedType}`);

// At runtime, property names follow camelCase despite TypeScript expectations.
Object.keys(obj).forEach(key => {
    console.log(`found property name: ${key}`);
});

Answer 2

What is the root cause of this problem? Is it related to TypeScript or Angular?

It's neither. The issue stems from the fact that the JSON data received from your server does not match the structure and format defined in the Info class in TypeScript.

Is there a specific convention I should adhere to?

Yes, there is. It's important to manually test and ensure that the data structures align with the classes you are casting them to. Before casting a JSON object to a specific class, parse it as a generic object and verify if it includes all the required properties matching the class structure (Info). Only then proceed with the casting process.

UPDATE: To address this issue, consider utilizing User-defined Typeguard functions in TypeScript. These functions can help determine whether an object belongs to a particular type, providing stronger assurance and type guarding capabilities. More information can be found here.

// User-defined type-guard function
function isInfo(obj: Object): obj is Info {
    if ('ManagedType' in obj && 'ApiTemplate' in obj) {
        return true;
    } else {
        return false;
    }
}

// Assume jsonString contains JSON data from an HTTP response body
let obj = JSON.parse(jsonString);

if (isInfo(obj)) {
    obj.ApiTemplate; // TypeScript recognizes obj as type Info in this scope
} else {
    // TypeScript knows obj is NOT of type Info in this scope
}

Why doesn't TypeScript flag the use of PascalCase as a potential issue?

TypeScript doesn't raise an error or warning because when using implicit cast like

this.HttpClient.get<Info[]>(this.Url).toPromise<Info[]>();

, you're essentially telling TypeScript that the returned JSON string will perfectly match the expected Info[] structure. However, due to case sensitivity differences, the actual runtime data may not align with the specified class definition. TypeScript relies on your implicit knowledge for such scenarios.

In essence:

You're converting a JSON object at runtime that may not entirely match the defined Info class due to property name casing discrepancies. This example highlights the issue:

//PascalCase
class Info 
{
    public ManagedType:string;

    public ApiTemplate:string;
}

let jsonString = `{
    "managedType": "1234asdf",
    "apiTemplate": "asdf1234"
}`;

// The following implicitly casts the JSON data to Info object assuming exact compatibility.
let obj: Info = JSON.parse(jsonString);

The above scenario mirrors the blind casting performed by

this.Infos = await this.HttpClient.get<Info[]>(this.Url).toPromise<Info[]>();

By utilizing PascalCase naming in the class definition but receiving camelCase properties in the JSON data, TypeScript cannot foresee these discrepancies during compilation and assumes your runtime conversions will align.

//Typescript expects PascalCase properties but they are actually camelCase - a discrepancy occurs at runtime.
console.log(`accessing Info.ManagedType property: ${obj.ManagedType}`);

// At runtime, property names follow camelCase despite TypeScript expectations.
Object.keys(obj).forEach(key => {
    console.log(`found property name: ${key}`);
});

Switching between PascalCase and camelCase in TypeScript leads to unexpected behavior

Answer №1

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