Code coverage basics in Node.js - Step-by-Step Execution

Concept Flow - Code coverage basics

Write code to test

↓

Write tests for code

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Run tests with coverage tool

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Coverage tool tracks executed lines

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Generate coverage report

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Review report to find untested code

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Improve tests to cover missing parts

↩Back to Run tests

This flow shows how code coverage works: write code, test it, run coverage, see report, then improve tests.

Execution Sample

Node.js

function sum(a, b) {
  if (a && b) {
    return a + b;
  }
  return 0;
}

// Test calls sum(1, 2) and sum(0, 2)

This code defines a sum function with a condition and tests it with two calls to check coverage.

Execution Table

Step	Code Line	Executed?	Reason	Coverage Status
1	function sum(a, b) {	Yes	Function declared	Covered
2	if (a && b) {	Yes	Test sum(1, 2) runs this true branch	Covered
3	return a + b;	Yes	sum(1, 2) returns 3	Covered
4	}	Yes	End if block	Covered
5	return 0;	Yes	Test sum(0, 2) runs this else branch	Covered
6	}	Yes	Function ends	Covered
7	// Test calls sum(1, 2) and sum(0, 2)	N/A	Comments not counted	N/A

💡 All code lines executed by tests, coverage is 100%

Variable Tracker

Variable	Start	After sum(1, 2)	After sum(0, 2)
a	undefined	1	0
b	undefined	2	2
return value	none	3	0

Key Moments - 2 Insights

Why does the coverage tool mark line 5 as covered even though it looks like an else?

Because the test sum(0, 2) calls the function with a=0, b=2, which makes the if condition false, so line 5 runs. See execution_table row 5.

Are comments counted in code coverage?

No, comments are ignored by coverage tools. See execution_table row 7 marked N/A.

Visual Quiz - 3 Questions

Look at the execution table, which line runs only when both a and b are truthy?

A Line 3 (return a + b)

B Line 5 (return 0)

C Line 2 (if condition)

D Line 1 (function declaration)

💡 Hint

Check rows 2 and 3 in execution_table where sum(1, 2) runs the true branch.

At which step does the coverage tool detect the else branch is executed?

A Step 2

B Step 3

C Step 5

D Step 1

💡 Hint

Look at execution_table row 5 where return 0 runs for sum(0, 2).

If we remove the test sum(0, 2), what happens to coverage on line 5?

A Line 5 remains covered

B Line 5 becomes uncovered

C Line 5 is ignored

D Line 5 coverage increases

💡 Hint

Refer to variable_tracker and execution_table rows showing line 5 runs only when sum(0, 2) is called.

Concept Snapshot

Code coverage tracks which lines run during tests.
Run tests with coverage tool to see report.
Report shows covered and uncovered code.
Improve tests to cover missing lines.
Helps find untested parts of code.

Full Transcript

Code coverage basics means checking which parts of your code run when you run tests. You write your code and tests, then run a coverage tool that tracks which lines execute. It creates a report showing covered and uncovered lines. For example, a sum function with an if condition can be tested with different inputs to cover both branches. Coverage helps you find code not tested so you can add tests and improve quality.