Answer by Tharindu Lakshan for Angular: conditional class with *ngClass

The directive operates in three different ways, depending on which of three types the expression evaluates to:

1. If the expression evaluates to a string, the string should be one or more space-delimited class names.
2. If the expression evaluates to an object, then for each key-value pair of the object with a truthy value the corresponding key is used as a class name.
3. If the expression evaluates to an array, each element of the array should either be a string as in type 1 or an object as in type 2. This means that you can mix strings and objects together in an array to give you more control over what CSS classes appear. See the code below for an example of this.

    [class.class_one] = "step === 'step1'"    [ngClass]="{'class_one': step === 'step1'}"

For multiple options:

    [ngClass]="{'class_one': step === 'step1', 'class_two' : step === 'step2' }"     [ngClass]="{1 : 'class_one', 2 : 'class_two', 3 : 'class_three'}[step]"    [ngClass]="step == 'step1' ? 'class_one' : 'class_two'"