Testing

Use TDD - Test Driven Development.
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Test code using unit testing strategy, i.e test only the new unit/code.

For example, if adding a battery to a car, test it by doing voltage test of the battery and not by starting the engine or driving the car.
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Test the whole system only when uploading product final prod version (end to end test).

You can use also flow tests to test certain flows in the platform.

Use frameworks for tests such as Jest for Javascript.

Debug

Use breakpoints, it is great (no need for print logs).
It shows the status of each variables.
You can add new breakpoints in each stop on a breakpoint (and then select continue).

You can set breakpoints using a code editor like VS Code or if you working on frontend of a website you can use Chrome dev tools (open dev tools -> source -> set breakpoint in specific file).

Here is an example of how it looks on VS Code:

Here we set breakpoints on lines 3, 8 and 9.
In order to set breakpoint just click with the mouse on the left side of the line.

After we run the code we see the panel for the debug session. You can continue after a breakpoint, stop the execution and restart the debug session.

Now the debug stopped in our breakpoint. Notice the variables on the left side.
Now, let's continue the debug session by pressing the continue button.

And here is the output result:

Mock Data

Use mock data in order to test apps.
You can use this website or even use this Python code:

s = ' '
for i in range (1, 20):
   s += 'INSERT INTO customer... VALUES (' + str(i) + ');\n\n'

print s

Bonus

To test with different emails that still belong to you, leverage Gmail aliases using the plus sign. Simply append a plus sign ("+") with any combination of letters or numbers after your email address. For instance, if your email is yourusername@gmail.com, you can use yourusername+friends@gmail.com or yourusername+mailinglists@gmail.com. This way, you'll receive emails at yourusername@gmail.com while organizing them based on the added identifier.