Combine [Ngstyle] with Condition (If. .Else)

One can also use this kind of condition:

<div [ngStyle]="myBooleanVar && {'color': 'red'}"></div>

It requires a bit less string concatenation...

The previous answers did not work for me, so I decided to improve this.

You should work with url(''), and not with value.

<li *ngFor="let item of items">
  <div
    class="img-wrapper"
    [ngStyle]="{'background-image': !item.featured ? 'url(\'images/img1.png\')' : 'url(\'images/img2.png\')'}">
  </div>
</li>

You can use this as follows:

<div [style.background-image]="value ? 'url(' + imgLink + ')' : 'url(' + defaultLink + ')'"></div>

To add and simplify Günter Zöchbauer's the example incase using (if...else) to set something else than background image :

<p [ngStyle]="value == 10 && { 'font-weight': 'bold' }">

<h2 [ngStyle]="serverStatus == 'Offline'? {'color': 'red'{'color':'green'}">Server with ID: {{serverId}} is {{getServerStatus()}} </h2>

or you can also use something like this:

<h2 [ngStyle]="{backgroundColor: getColor()}">Server with ID: {{serverId}} is {{getServerStatus()}}</h2>

and in the *.ts

getColor(){return this.serverStatus === 'Offline' ? 'red' : 'green';}

I have used the below code in my existing project

<div class="form-group" [ngStyle]="{'border':isSubmitted && form_data.controls.first_name.errors?'1px solid red':'' }">
</div>

Trying to set background color based on condition:   

Consider variable x with some numeric value.    
<p [ngStyle]="{ backgroundColor: x > 4 ? 'lightblue' : 'transparent' }">
        This is a sample Text
</p>

[ngStyle] with condition based if and else case.

<label for="file" [ngStyle]="isPreview ? {'cursor': 'default'} : {'cursor': 'pointer'}">Attachment

Use:

[ngStyle]="{'background-image': 'url(' +1=1 ? ../../assets/img/emp-user.png : ../../assets/img/emp-default.jpg + ')'}"