<section [ngClass]="{'class1':condition1, 'class2': condition2, 'class3':condition3}" > 

<!-- notice expression1 used twice -->

<div ng-class="{class1 : expression1, class2 : expression1}">

    Hello World!

</div>

[ngClass]="[

    expression1 ? 'class1 class2' : 'class3 class4',

    expression2 ? 'class5' : 'class6',

    ...

]"

[ngClass]="[ mainIMG.touched && !mainIMG.valid? 

            'class1 class2':

            'class3 class2' ]"

<div ng-class="{class1 : expression1, class2 : expression2}">

    Hello World!

</div>

​