Angular is a platform and framework for building single-page client applications using HTML and TypeScript.
The brackets, , cause Angular to evaluate the right-hand side of the assignment as a dynamic expression.
Without the brackets, Angular treats the right-hand side as a string literal and sets the property to that static value.
The right-hand side of the assignment
[ngClass]="classes" is a property, not a string.
classes = "class-1 class-2 class-3";
<p [ngClass]="classes">Brackets demo</p> // output // <p class="class-1 class-2 class-3">Brackets demo</p>