Code coverage reporting in Deno - Step-by-Step Execution

Concept Flow - Code coverage reporting

Write tests for code

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Run tests with coverage flag

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Deno collects coverage data

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Generate coverage report

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Review coverage report

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Improve tests if coverage low

↩Back to Write tests

This flow shows how Deno runs tests with coverage enabled, collects data, generates a report, and helps improve test quality.

Execution Sample

Deno

deno test --coverage=coverage_data

deno coverage coverage_data --lcov > coverage.lcov

deno coverage coverage_data --json > coverage.json

Run tests with coverage, then generate coverage reports in lcov and json formats.

Execution Table

Step	Action	Input/Command	Output/Result
1	Run tests with coverage flag	deno test --coverage=coverage_data	Tests run, coverage data saved in 'coverage_data' directory
2	Generate lcov report	deno coverage coverage_data --lcov > coverage.lcov	LCOV format coverage report created in 'coverage.lcov'
3	Generate JSON report	deno coverage coverage_data --json > coverage.json	JSON format coverage report created in 'coverage.json'
4	Review coverage	Open coverage.lcov or coverage.json	See which lines are covered or missed by tests
5	Improve tests if needed	Add or modify tests	Better coverage on uncovered code
6	Re-run tests with coverage	deno test --coverage=coverage_data	Updated coverage data collected
7	Repeat report generation	deno coverage coverage_data --lcov > coverage.lcov	Updated coverage report
8	Exit	Coverage satisfactory or complete	Stop improving tests

💡 Coverage is satisfactory or no more tests to add, so process stops.

Variable Tracker

Variable	Start	After Step 1	After Step 6	Final
coverage_data	empty	collected coverage info from tests	updated coverage info after re-run	final coverage data used for reports
tests	written but unrun	run with coverage collected	run again after improvements	final test suite with coverage

Key Moments - 3 Insights

Why do we run tests twice in the flow?

The first run collects initial coverage data (see Step 1). After reviewing the report (Step 4), we improve tests and run again (Step 6) to update coverage.

What formats can coverage reports be generated in?

Deno can generate coverage reports in lcov and json formats as shown in Steps 2 and 3.

Where is coverage data stored during the process?

Coverage data is stored in the directory specified by --coverage flag, here 'coverage_data' (Step 1 and Step 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the output after Step 1?

A Tests fail without coverage data

B Coverage report generated in JSON format

C Tests run and coverage data saved in 'coverage_data'

D No tests run

💡 Hint

Check Step 1 output column in the execution_table.

At which step do we generate the lcov coverage report?

A Step 2

B Step 4

C Step 3

D Step 6

💡 Hint

Look for 'lcov' in the Input/Command column in execution_table.

If we skip improving tests after reviewing coverage, what happens next?

A We re-run tests to update coverage data

B The process stops at Step 4

C We generate coverage reports again without new data

D Coverage data is deleted

💡 Hint

Refer to the exit_note and Step 4 in execution_table.

Concept Snapshot

Code coverage in Deno:
- Run tests with --coverage flag to collect data
- Use 'deno coverage' to generate reports (lcov, json)
- Review reports to see tested lines
- Improve tests and re-run for better coverage
- Repeat until coverage is satisfactory

Full Transcript

Code coverage reporting in Deno involves running your tests with a special flag that collects information about which parts of your code are tested. After running tests, you generate reports in formats like lcov or json to see which lines were covered. If coverage is low, you add or improve tests and run again to update the coverage data. This cycle helps ensure your code is well tested and reliable.

Practice

1. What does the deno test --coverage command do?

easy

A. Installs coverage tools for Deno

B. Only runs tests without coverage data

C. Generates a coverage report without running tests

D. Runs tests and collects code coverage data

Solution

1. Step 1: Understand the purpose of deno test --coverage

   This command runs your test files and collects data on which parts of your code were executed during testing.

2. Step 2: Differentiate from other commands

   Unlike just running tests, this command also tracks coverage. It does not generate reports directly; that is done by deno coverage.

3. Final Answer:

   Runs tests and collects code coverage data -> Option D

4. Quick Check:

   deno test --coverage collects coverage data [OK]

Hint: Remember: test runs + coverage data collection together [OK]

Common Mistakes:

• Thinking it only runs tests without coverage
• Confusing coverage data collection with report generation
• Assuming it installs tools instead of running tests

2. Which of the following is the correct command to view a coverage report in the terminal?

easy

A. deno coverage --reporter=text

B. deno test --view-coverage

C. deno coverage --show

D. deno test --coverage-report

Solution

1. Step 1: Identify the command to view coverage

   The deno coverage command is used to view or export coverage reports after running tests with coverage.

2. Step 2: Recognize the correct flag for terminal output

   The flag --reporter=text tells Deno to show the coverage report in the terminal.

3. Final Answer:

   deno coverage --reporter=text -> Option A

4. Quick Check:

   Use deno coverage with --reporter=text to view coverage [OK]

Hint: Use deno coverage with --reporter=text to see output [OK]

Common Mistakes:

• Using deno test flags to view coverage report
• Assuming --show or --view-coverage are valid flags
• Confusing test and coverage commands

3. Given the following commands run in order:

deno test --coverage=./cov_data
deno coverage ./cov_data --reporter=lcov

What is the expected output?

medium

A. Tests run but no coverage data is saved

B. A coverage report file in lcov format is generated

C. An error occurs because coverage directory is missing

D. Coverage report is printed directly in the terminal

Solution

1. Step 1: Run tests with coverage saved to a directory

   The first command runs tests and saves coverage data into the folder ./cov_data.

2. Step 2: Generate coverage report in lcov format

   The second command reads coverage data from ./cov_data and outputs a report in lcov format to a file (stdout is the report content, but typically redirected to a file).

3. Final Answer:

   A coverage report file in lcov format is generated -> Option B

4. Quick Check:

   Coverage data saved + lcov report generated [OK]

Hint: Save coverage data first, then generate report from it [OK]

Common Mistakes:

• Expecting coverage report without saving data first
• Thinking lcov generates a file automatically (use redirection)
• Assuming error if directory exists

4. You ran deno test --coverage but deno coverage shows no data. What is the likely cause?

medium

A. Your test files have syntax errors

B. You forgot to specify a coverage directory when running tests

C. You ran deno coverage before running tests

D. Coverage data is automatically deleted after tests

Solution

1. Step 1: Understand coverage data flow

   Coverage data is collected when tests run with coverage enabled and saved to a directory or default location.

2. Step 2: Check command order

   If deno coverage is run before tests, no coverage data exists yet, so it shows no data.

3. Final Answer:

   You ran deno coverage before running tests -> Option C

4. Quick Check:

   Run tests first, then coverage report [OK]

Hint: Always run tests with coverage before viewing reports [OK]

Common Mistakes:

• Assuming coverage directory is mandatory every time
• Thinking syntax errors cause no coverage data
• Believing coverage data auto-deletes after tests

5. You want to generate a coverage report in HTML format and open it in your browser. Which sequence of commands achieves this correctly?

hard

A. deno test --coverage=./cov_data && deno coverage ./cov_data --reporter=html && open coverage/index.html

B. deno coverage --reporter=html && deno test --coverage && open ./cov_data/index.html

C. deno test && deno coverage --reporter=html && open ./cov_data/index.html

D. deno test --coverage && deno coverage --reporter=html > report.html && open report.html

Solution

1. Step 1: Run tests with coverage saved to a directory

   Use deno test --coverage=./cov_data to run tests and save coverage data in ./cov_data.

2. Step 2: Generate HTML report from saved coverage data

   Run deno coverage ./cov_data --reporter=html to create HTML files inside coverage/ directory.

3. Step 3: Open the generated HTML report in browser

   Use open coverage/index.html (macOS) or equivalent to view the report.

4. Final Answer:

   deno test --coverage=./cov_data && deno coverage ./cov_data --reporter=html && open coverage/index.html -> Option A

5. Quick Check:

   Save coverage, generate HTML, then open file [OK]

Hint: Save coverage, generate HTML report, then open it [OK]

Common Mistakes:

• Generating report before running tests
• Not specifying coverage directory consistently
• Redirecting HTML output to file instead of directory