Exploring TypeScript's type checking functionality

Question

Exploring TypeScript's type checking functionality

I've been pondering about the concept of type guards in TypeScript, particularly regarding their necessity when only one type is defined in a method signature. Most examples in the TypeScript documentation focus on scenarios involving union types, like this:

myMethod(argument: string | number) {
 if (typeof argument === 'string') { // do something }
 if (typeof argument === 'number') { // do something }

However, I've noticed people utilizing typeof even with strongly typed parameters, as seen here:

myMethod(argument: string) {
 if (typeof argument === 'string') { // so something }

What are your thoughts on this approach? How do you handle data validation, especially for information obtained from external sources like API endpoints?

angular typescript

Answer 1

Answer №1

if your code looks like this

myMethod(argument: string) {

then there is no need to check the type explicitly, as TypeScript will throw an error if you try to assign a different type to a variable, just like this:

let strVar : string;
strVar = 10;//error (you can't assign anything other than a string type)

In Angular, you can prevent implicit conversion to any type by configuring your options like this:

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "moduleResolution": "node",
    "sourceMap": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "lib": [ "es2015", "dom" ],
    "noImplicitAny": true,//prevent implicit conversion to any type
    "suppressImplicitAnyIndexErrors": true
  }
}

For more details, visit: https://angular.io/guide/typescript-configuration

Answer 2

if your code looks like this

myMethod(argument: string) {

then there is no need to check the type explicitly, as TypeScript will throw an error if you try to assign a different type to a variable, just like this:

let strVar : string;
strVar = 10;//error (you can't assign anything other than a string type)

In Angular, you can prevent implicit conversion to any type by configuring your options like this:

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "moduleResolution": "node",
    "sourceMap": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "lib": [ "es2015", "dom" ],
    "noImplicitAny": true,//prevent implicit conversion to any type
    "suppressImplicitAnyIndexErrors": true
  }
}

For more details, visit: https://angular.io/guide/typescript-configuration

Answer 3

Answer №2

When encountering unfamiliar objects with custom interfaces, it is recommended to use the is keyword along with a typeguard function.

For instance, if we want to determine whether plant is of type IFruit or IVegetable,

function isVeggie(plant: any): plant is IVeggie {
  return p.vegetable !== undefined;  // vegetable is a property on IVeggie
}
function isFruit(plant: any): plant is IFruit {
  return p.fruit !== undefined;  // fruit is a property on IFruit
}

The type guards should yield either true or false, checking for specific properties on each type for validation.

To implement these type guards, casting to any is required.

if (isVeggie(plant)) {
   plant as any as IVeggie;
   // Confirms that plant is indeed of type IVeggie
}

It's worth noting that you must first assert toany before asserting to a custom interface like IVeggie. This process may be slightly more complex compared to other programming languages.

Answer 4

When encountering unfamiliar objects with custom interfaces, it is recommended to use the is keyword along with a typeguard function.

For instance, if we want to determine whether plant is of type IFruit or IVegetable,

function isVeggie(plant: any): plant is IVeggie {
  return p.vegetable !== undefined;  // vegetable is a property on IVeggie
}
function isFruit(plant: any): plant is IFruit {
  return p.fruit !== undefined;  // fruit is a property on IFruit
}

The type guards should yield either true or false, checking for specific properties on each type for validation.

To implement these type guards, casting to any is required.

if (isVeggie(plant)) {
   plant as any as IVeggie;
   // Confirms that plant is indeed of type IVeggie
}

It's worth noting that you must first assert toany before asserting to a custom interface like IVeggie. This process may be slightly more complex compared to other programming languages.

Exploring TypeScript's type checking functionality

Answer №1

Answer №2

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