you need object notation

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

ref: NgClass

There are multiple ways to write same logic. As it mentioned earlier, you can use object notation or simply expression. However, I think you should not that much logic in HTML. Hard to test and find issue. You can use a getter function to assign the class.

For Instance;

public getCustomCss() {
    //Logic here;
    if(this.x == this.y){
        return 'class1'
    }
    if(this.x == this.z){
        return 'class2'
    }
}
<!-- HTML -->
<div [ngClass]="getCustomCss()"> Some prop</div>

//OR

get customCss() {
    //Logic here;
    if(this.x == this.y){
        return 'class1'
    }
    if(this.x == this.z){
        return 'class2'
    }
}
<!-- HTML -->
<div [ngClass]="customCss"> Some prop</div>

<section [ngClass]="{'class1': expression1, 'class2': expression2, 
'class3': expression3}">

Don't forget to add single quotes around class names.

<a [ngClass]="{'class1':array.status === 'active','class2':array.status === 'idle','class3':array.status === 'inactive',}">

you can use specific class instead of use [ngClass] .Please note that in the second example I have used the age variable as an expiation purpose .

Example 1:

<section
    [class.class1]="menu1"
    [class.class2]="menu2"
    [class.class3]="menu3">
    </section>

Example 2:

   <section
    [class.class1]="age > 20"
    [class.class2]="age < 50"
    [class.class3]="age == 4">
    </section>

I had this similar issue. I wanted to set a class after looking at multiple expressions. ngClass can evaluate a method inside the component code and tell you what to do.

So inside an *ngFor:

<div [ngClass]="{'shrink': shouldShrink(a.category1, a.category2), 'showAll': section == 'allwork' }">{{a.listing}}</div>

And inside the component:

section = 'allwork';

shouldShrink(cat1, cat2) {
    return this.section === cat1 || this.section === cat2 ? false : true;
}

Here I need to calculate if i should shrink a div based on if a 2 different categories have matched what the selected category is. And it works. So from there you can computer a true/false for the [ngClass] based on what your method returns given the inputs.

I hope this is one of the basic conditional classes

Solution: 1

<section [ngClass]="(condition)? 'class1 class2 ... classN' : 'another class1 ... classN' ">

Solution 2

<section [ngClass]="(condition)? 'class1 class2 ... classN' : '(condition)? 'class1 class2 ... classN':'another class' ">

Solution 3

<section [ngClass]="'myclass': condition, 'className2': condition2">