How do you initialize matSortDirection?-CodePudding

I am working through an example in an ASP.Net Core 6 and Angular 13 book on Material Tables, paging, sorting and filtering.

The material table in my angular component html looks like this:

<table mat-table [dataSource]="cities"
   
   [hidden]="!cities"
   matSort (matSortChange)="loadData()"
   [matSortActive]="defaultSortColumn"
   [matSortDirection]="defaultSortOrder">

And in the component ts file or the controller, it has defaultSortOrder initialized like this:

public defaultSortOrder: "asc" | "desc" = "asc";

I don't understand this syntax. I read it as: "asc" OR "desc" is assigned "asc".

It seems to work but I don't understand what is going on here. If I try to initialize it to just "asc",

public defaultSortOrder = "asc";

it says: "Type 'string' is not assignable to type 'SortDirection'."

I tried to read the documentation but I still don't get it.

What does this syntax mean?

public defaultSortOrder: "asc" | "desc" = "asc";

CodePudding user response：

The code "asc" | "desc" is simply declaring the type for defaultSortOrder. It is specifying a union of string literal types, which just means the type is a string of either 'asc' or 'desc'.

The code = "asc"; is simply assigning the initial value to defaultSortOrder.

The reason you receive an error when you omit the type is because Typescript infers the type from the assigned value as any string but the component requires a narrower type of a string with values'asc' | 'desc'.

So instead of reading as

"asc" OR "desc" is assigned "asc".

Read as:

A variable allowed to have the values "asc" or "desc" is assigned "asc"

CodePudding user response：

The type can be either of the two values. If you set the value as 'test' you will get a type error.

this.defaultSortOrder = "test"; // <- type error!

So we can use this method of defining, when you want the property to have only certain values, in this case asc or desc!