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[ngClass]="[ mainIMG.touched && !mainIMG.valid?
'class1 class2':
'class3 class2' ]"
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<section [ngClass]="{'class1':condition1, 'class2': condition2, 'class3':condition3}" >
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[ngClass]="[
expression1 ? 'class1 class2' : 'class3 class4',
expression2 ? 'class5' : 'class6',
]"