On the Facebook site or its mobile version, it is possible to prevent others from seeing your friends list.
Steps
Method 1 of 2: Hide friends on mobile

Step 1. Open Facebook
The app is dark blue with a white "f". If you are already logged in to Facebook, your news feed will appear.
If you are not logged in, enter your email address (or phone number) and password before continuing

Step 2. Press ☰
This option can be found either in the lower right corner of the screen (on iPhone) or at the top right (on Android).

Step 3. Scroll down and choose Settings
Skip this step if you are using an Android.

Step 4. Select Account Settings
This will open the Facebook settings.

Step 5. Tap Privacy
This option is near the top of the screen.

Step 6. Select Who can see your Friends list?
You will find this option near the top of the screen.

Step 7. Choose Me Only
This option is located in the middle of the screen. Once you click on this option, your friends list will not be visible to other people.
Method 2 of 2: Hide friends on a computer

Step 1. Go to the Facebook site
To do this, open the page https://fr-fr.facebook.com in your browser. If you are already logged in, your Facebook news feed will appear.
If you are not logged in, enter your email address (or phone number) and password before continuing

Step 2. Click ▼
This button is located in the upper right corner of the page, just to the right of the symbol ?

Step 3. Select Settings
This option is near the bottom of the drop-down menu.

Step 4. Choose Privacy
This is a tab located at the top left of the page Settings.

Step 5. Select Who can see your Friends list?
This is the second option in the section How others can find and contact you.

Step 6. Click the box
This box, which is at the bottom of the section Who can see your friend list?, will probably display Friends Where Public.

Step 7. Choose Me Only
This option is located towards the middle of the drop-down menu. Now only you will be able to see your friend list.