Angular 6 *ngFor display different styles for first, odd, even and last

user_7311894 0
Viewed 8722

I am learning Angular 6 and just trying to put togheter some of the stuff I have learned and I am currently running into an issue that I cannot find an answer to. I am trying to change the style of a LI using *ngFor depending if the index is First, Last, Odd or Even. So far everything works but I can't figure out how to do it for the Last because everything I add a new object to my list, it is obviously the last so it render the color for the last.

I understand how to do it but the real problem is that I am adding stuff dynamicly to my list from a form and I'm not sure how to evaluate the Last so that the others become to right color.

Keep in mind that I am still a newb and it might look messy and I also understand that some client-side validations I am doing are probably not optimal or required since HTMl5 but I made it to learn.

Here is my code for my component HTML

>

<h1>List of courses :</h1><br>
<div *ngIf="courses.length > 0; then coursesList else noCourses"></div>

<ng-template #coursesList>
  <h2>List of Courses :</h2>
  <ul *ngFor="let course of courses; index as i;">
    <li [ngStyle]="{'background-color':getColor(i)}" style="color: white;">
      <strong>Index : </strong>{{i}} <strong>ID : </strong>{{course.id}} <strong>Name</strong> : {{course.name}}
      <button (click)="onRemove(i)">Remove</button>
      <button (click)="onModify(i)">Modify</button>
    </li>
  </ul>
</ng-template>
<ng-template #noCourses>
  <h5>There are no courses in this list. Use the form bellow to add some.</h5>
</ng-template>

<div (keyup.enter)="onAdd()">
  <span>ID : <input type="number" (keypress)="checkNumber($event)" [(ngModel)]="fields.id" placeholder="Enter an ID"></span>
  <span>Name : <input type="text" [(ngModel)]="fields.name" placeholder="Enter a NAME"></span>
  <button (click)="onAdd()">Add</button>
  <button (click)="onClear()">Clear</button>
</div>
<div *ngIf="isNotNumber" style="background-color: red; color:black"><strong>ID can only be numbers !</strong></div>
<div *ngIf="noValues" style="background-color: red; color:black"><strong>Please fill all fields !</strong></div>
<div *ngIf="noModifyValues" style="background-color: red; color:black"><strong>To modify enter all informations!</strong></div>

Code for .TS

>

import { Component } from '@angular/core';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})


export class AppComponent {
  noValues: boolean;
  noModifyValues: boolean;
  isNotNumber: boolean;
  fields: Courses = {id: null, name: null};
  courses: Array<Courses> = [];
  viewMode: string = null;

  checkNumber($event) {
    if ($event.keyCode != 13) {
      isFinite($event.key) ? this.isNotNumber = false : this.isNotNumber = true;
    }
  }

  onAdd() {

    if (!this.fields.id || !this.fields.name) {
      this.noValues = true;
    } else {
      this.courses.push({id: this.fields.id, name: this.fields.name});
      this.fields.id = null;
      this.fields.name = null;
      this.noValues = false;
    }
  }

  onRemove(i) {
    this.courses.splice(i, 1);
  }

  onClear() {
    this.courses = [];
    this.fields.id = null;
    this.fields.name = null;
    this.noValues = false;
  }

  onModify(i) {
    if (!this.fields.id || !this.fields.name) {
      this.noModifyValues = true;
    } else {
      this.courses[i].name = this.fields.name;
      this.courses[i].id = this.fields.id;
      this.noModifyValues = false;

    }
  }

  getColor(i){
    if (i % 2 === 0 && i != 0){i = 'odd';}
    switch (i) {
      case i = 0 : return 'orange';
      case i = 'odd' : return 'blue';
    }
    return 'red';
  }
}
interface Courses {
  id: number;
  name: string;
}

Image of the code in action for better understanding.

angular6 angular typescript arrays
4 Answers

If you only want change the background-color you can use [style.background-color] and you can use ternary operator in the .html

<ul *ngFor="let course of courses; let index=i;
                                   let odd=odd;
                                   let last=last;
                                   let first=first">
    <li [style.backgound-color]="first?'orange':last?'purple':odd?'blue':'red'">
          ...
    </li>
</ul>
user_85581860

Try something like this

getColor(i){
    if (i % 2 === 0 && i != 0){i = 'odd';}
    if (this.courses && (this.courses.length - 1 === i)) {i = 'last'}
    switch (i) {
      case i = 0 : return 'orange';
      case i = 'odd' : return 'blue';
    }
    return 'red';
  }

Hope it works - Happy coding !!

user_83986300

Thanks Rahul. The part I was missing is evaluating if there is something in courses. However, I had to had a few more lines to Odd and Last as follow :

getColor(i){
    if (this.courses && i != 0 && (this.courses.length - 1 === i)) {i = 'last'}
    if (i % 2 === 0 && i != 0 && i != 'last'){i = 'odd';}
    switch (i) {
      case i = 0 : return 'orange';
      case i = 'odd' : return 'blue';
      case i = 'last' : return 'purple';
    }
    return 'red';
  }

Quick question. It seems like a whole lot of IF and && and checking specific things. Is that the way to do it properly?

user_73118940

You could use if else ladder instead of mixing up if else and switch and assignments like given below

getColor(i)
{
  if(this.courses)
  {
  if(i==0)
      return "orange";
  else if(i==this.courses.length-1)
      return "purple";
  else if (i%2==0)
      return "red";
  else
      return "blue";
  }
}
user_43693740
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