29.2. ngFor

In the Angular lesson 1 exercises, you modified a movie-list component to display a series of titles. The final code within movie-list.component.html probably looked something like:

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<div class='movies'>
   <h3>Movies to Watch</h3>
   <ol>
      <li>{{movies[0]}}</li>
      <li>{{movies[1]}}</li>
      <li>{{movies[2]}}</li>
      <li>{{movies[3]}}</li>
   </ol>
</div>

movies[0] - movies[3] reference an array assigned within the movie-list.component.ts file.

To change the number of movie titles displayed in the ordered list, we could manually add or remove li tags, or we could use the structural directive ngFor to iterate through the movie options.

29.2.1. ngFor Syntax

The example below shows the basic approach for using ngFor to iterate through the contents of an array. For a more detailed guide to using ngFor and all of its variations, refer to the following resources:

  1. Angular documentation,
  2. Malcoded website.

Just like a for loop in JavaScript requires a specific syntax in order to operate, loops in Angular must follow a set of rules. Let's explore these rules by adding ngFor to our movie list code.

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<div class='movies'>
   <h3>Movies to Watch</h3>
   <ol>
      <li *ngFor ="let movie of movies">{{movie}}</li>
   </ol>
</div>

Some items to note:

  1. Structural directives all begin with the * symbol.
  2. The string "let movie of movies" provides the instructions for running the loop.
    1. The let keyword declares the movie variable.
    2. of movies sets movie equal to the first element of the movies array. Each iteration of the loop sets movie equal to the next title in the array.
  3. The *ngFor statement is placed INSIDE the <li> tag.
  4. {{movie}} is the placeholder for the current value of movie.

By placing the *ngFor statement inside the tag, the loop generates multiple <li></li> elements. Each iteration adds a new list item to the HTML code, one for each title in the movies array.

Warning

The *ngFor statement generates a new HTML tag for each item in the array. Be careful where you put the statement! If we had added *ngFor = "let movie of movies" to the <h3> tag, then the Movies To Watch title would have been repeated multiple times.

In general, the syntax for *ngFor is:

*ngFor = "let variableName of arrayName"

Where variableName is the loop variable, and arrayName represents the array to iterate through.

Note

*ngFor only operates over the contents of an array. If we want to iterate over the characters in a string, we must first convert it into an array.

There is a technique for iterating over the key/value pairs of an object, but that is a more advanced topic. We will not discuss that method here.

29.2.2. Try It

From the lesson2 folder in VSCode, open the examples/ngfor-practice/src/app/chores folders and select the chores.component.html file.

Access ngFor practice in VSCode.

The starter code should match this:

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<div class='chores'>
   <h3>Chores To Do Today</h3>
   <ul>
      <li>{{chores[0]}}</li>
      <li>{{chores[1]}}</li>
      <li>{{chores[2]}}</li>
   </ul>
   <hr>
</div>

In the VSCode terminal window, navigate to the ngfor-practice folder.

$ pwd
   angular-lc101-projects/lesson2
$ ls
   examples        exercises
$ cd examples
$ ls
   input-practice  ngfor-practice  ngif-practice
$ cd ngfor-practice

Once you are in the folder, enter npm install in the terminal. This will add all of the Angular modules needed to run the project.

Enter ng serve to launch the project, then:

  1. Modify chores.component.html with *ngFor to loop over the chores array:

    1. Replace line 4 with <li *ngFor = "let chore of chores">{{chore}}</li>.
    2. Delete lines 5 and 6.
    3. Save your changes.
    4. Reload the web page to verify that all the chores are displayed.
  2. Open chores.component.ts. Add "Clean bathroom" to the chores array, then save. Reload the web page to make sure the new chore appears. Your output should look like this:

    *ngFor first solution.
  3. Remove two chores from the array. Reload the web page to make sure these items disappear from the list.

  4. Use *ngFor within the <div> tag to loop over the todoTitles array:

    1. Replace line 1 with <div class='chores' *ngFor = "let title of todoTitles">.
    2. Replace "Chores To Do Today" in line 2 with a placeholder for title.
    3. Save your changes, then reload the page. Properly done, your page should look something like:
    *ngFor practice solution.
  5. Return to chores.component.ts. Add an item to the todoTitles array, then save. Check to make sure another list appears on the web page. Next, remove two items from the todoTitles array. Save and make sure the page reflects the changes.

29.2.2.1. What If

  1. What if you placed the *ngFor statement inside the <h3> tag instead of the <div> tag? Try it and see what happens!
  2. What if you placed the statement inside the <ul> tag instead? Try it!

29.2.2.2. Bonus What If

What if we want to have different chores listed for Yesterday, Today, and Tomorrow?

*ngFor bonus solution.

Accomplishing this task is OPTIONAL, but it boosts your skill level and makes your page look better.

  1. In the chores.component.ts file, replace the chores and todoTitles arrays with the following array of objects:

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    chores = [
       {title: "Yesterday's Chores", tasks: ['Empty dishwasher', 'Start LaunchCode prep work', 'Buy groceries']},
       {title: "Today's Chores", tasks: ['Load dishwasher', 'Finish LaunchCode prep work', 'Buy the groceries you forgot']},
       {title: "Tomorrow's Chores", tasks: ['Empty dishwasher AGAIN', 'Play with LaunchCode practice code', 'Groceries AGAIN']},
    ]
    
  2. Update line 1 in chores.component.html to access each object in the chores array:

    1. <div class='chores' *ngFor = 'let list of chores'>
    2. Each iteration, list will be assigned a new object with title and tasks properties.
  3. Update the placeholder in line 2 to access the title property of list.

  4. Update line 4 to loop over the tasks array: <li *ngFor = 'let chore of list.tasks'>.

29.2.3. Check Your Understanding

The following questions refer to this code sample:

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<div>
   <h3>My Pets</h3>
   <ul>
      <li>{{pet}}</li>
   </ul>
</div>

Assume that we have defined a pets array that contains 4 animals.

Question

Adding *ngFor = 'let pet of pets' to the <li> tag produces:

  1. 4 headings
  2. 4 unordered lists
  3. 4 list items
  4. 4 headings each with 4 list items

Question

Moving *ngFor = 'let pet of pets' from the <li> tag to the <div> tag produces:

  1. 1 heading and 4 unordered lists with 4 pets each
  2. 4 headings and 4 unordered lists with 4 pets each
  3. 1 heading and 4 unordered lists with 1 pet each
  4. 4 headings and 4 unordered lists with 1 pet each