add a class value using ngif in angular 6 code example

Example 1: Angular 8 ngClass If

[ngClass]="{'my-class': step=='step1'}"

Example 2: conditionally assign a css class angular 9

<div [ngClass]="{'message': info.priority < 10,
                 'warn': info.priority > 10 && info.priority < 20, 
                 'error': info.priority > 30}">

{{info.text}}

</div>