This repository contains various experiments with TypeScript.
If you have ever worked with JSON in an Angular application, you know that you cannot
trust the deserialized results. JavaScript doesn't have Reflection, JSON doesn't have
a data type for a Date and so... JSON.parse doesn't know that you want the
incoming string value "2018-07-15T05:35:03.000Z" converted into a Date.
Say you have defined an interface Order as:
export interface Order {
orderId?: number;
orderNumber?: string;
pickupDateTime?: Date
}You call your Angular HttpClient with the generic HttpClient#get<T> overload:
this.httpClient
.get<Order>(url, ...)
.pipe(
map((response: Order) => {
// Work with the Response ...
})
);Your Webservice responds with a totally valid JSON Order as:
{
"orderId": 1,
"orderNumber": "8472-423-14",
"pickupDateTime":"2018-07-15T05:35:03.000Z"
}And with a simple test, we can see, that the returned pickupDateTime is actually a string and not a Date:
describe('JSON.parse', () => {
it('Should not parse string as Date', () => {
// prepare
const json = `
{
"orderId": 1,
"orderNumber": "8472-423-14",
"pickupDateTime":"2018-07-15T05:35:03.000Z"
}`;
// act
const verifyResult = JSON.parse(json) as Order;
// verify
const isTypeOfString = typeof verifyResult.pickupDateTime === "string";
const isInstanceOfDate = verifyResult.pickupDateTime instanceof Date;
deepStrictEqual(isTypeOfString, true);
deepStrictEqual(isInstanceOfDate, false);
});
});In this example we will take a look at providing a set of decorators (@JsonProperty, @JsonType, ...) to
decorate classes and deserialize the JSON into the correct types. It will look like this:
export class Order {
@JsonProperty("orderId")
orderId?: number;
@JsonProperty("orderNumber")
orderNumber?: string;
@JsonProperty("pickupDateTime")
@JsonType(Date)
pickupDateTime?: Date;
}And with a simple test, we will see the correct types on our deserialized object:
describe('deserialize', () => {
it('Decorated Order should parse Date', () => {
// prepare
const json = `
{
"orderId": 1,
"orderNumber": "8472-423-14",
"pickupDateTime":"2018-07-15T05:35:03.000Z"
}`;
// act
const verifyResult: Order = deserialize<Order>(json, Order);
// verify
const isTypeOfObject = typeof verifyResult.pickupDateTime === "object";
const isInstanceOfDate = verifyResult.pickupDateTime instanceof Date;
deepStrictEqual(isTypeOfObject, true);
deepStrictEqual(isInstanceOfDate, true);
});
});That enables us a bit more type-safety, when deserializing incoming JSON data.