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<div [ngClass]="{className: expresion }">
  Sarasa
</div>

<!--The expresion is evaluated and if is true the class will be add to the element -->

<div [ngClass]="{className: true }">
  Sarasa
</div>

<div [ngClass]="{className: 1 + 1 }">
  Sarasa
</div>

<div [ngClass]="{className: propertyThatIsTRue }">
  Sarasa
</div>

<div [ngClass]="{'red': 1+1 != 0}">
  Sarasa
</div>
  Sarasa
</div>