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V8 coverage

This repository contains helper functions to manipulate V8 coverage files.

  • Stable features:
    • Clone: Perform deep copies of V8 coverages
    • Merge: Merge multiple V8 coverages into a single one. The merge does not rely on the source text.
  • Unstable:
    • Normalization: Normalize a V8 coverage by sorting it's item and fusing some ranges. Requires more tests before being exposed.
  • Planned (unavailable yet):
    • Test validity: Check if a JS object is a valid V8 coverage.
    • Diff: Opposite of merge: get the difference between two coverages
    • Test "inclusion": Verify if two coverages are compatible (one "includes" the other).

There are three implementations:

  • Typescript (ts): Reference implementation, available on npm as @c88/v8-coverage.
  • Rust (rs): Available on as v8-coverage.
  • Node native module (node): Not available yet, planned to be published on npm as @c88/v8-coverage-node.

All the implementations expose the same API.


Support policy: Rust version 1.56.0 or later.


Support policy: Node version 14.13.1 or later.


mergeProcessCovs(processCovs: ProcessCov[]): ProcessCov

Merges multiple process coverages into a single process coverage.

mergeScriptCovs(scriptCovs: ScriptCov[]): ScriptCov | undefined

Merges a list of matching script coverages into a single script coverage.

The lib assumes two scripts are matching if they have the same url.

If the list is empty, returns undefined.

mergeFunctionCovs(funcCovs: FunctionCov[]): FunctionCov | undefined

Merges a list of matching function coverages into a single function coverage.

The lib assumes two scripts are matching if their "root range" (first range) has the same offsets.

If the list is empty, returns undefined.

cloneProcessCov(processCov: ProcessCov): ProcessCov

cloneScriptCov(functionCov: FunctionCov): FunctionCov

cloneFunctionCov(scriptCov: ScriptCov): ScriptCov

cloneRangeCov(rangeCov: RangeCov): RangeCov



Coverage results for a single process. It contains a list of script coverages. There is one entry per script.

interface ProcessCov {
  result: ScriptCov[];
pub struct ProcessCov {
  pub result: Vec<ScriptCov>,


  • Inside a single ProcessCov, the scriptId values are unique.
  • Between multiple ProcessCov, scripts with the same url do not always have the same scriptId. The scriptId depends on the load order, so dynamically loading modules may cause different scriptId values.


  • Inside a single ProcessCov, the url values can be non-unique. I haven't checked it yet but it seems possible if you load a module, then clear it from cache, overwrite its file and reload it. Another way to achieve it may be using the vm module.


Coverage data for a single script. A script can be a CJS module, an ES module, a Node internal module, a dynamic module (for example cjs-bridge:// modules), etc. It holds a list of function coverages, for each of its functions.

interface ScriptCov {
  scriptId: string;
  url: string;
  functions: FunctionCov[];
pub struct ScriptCov {
  pub script_id: String,
  pub url: String,
  pub functions: Vec<FunctionCov>,


  • Inside a single ScriptCov, the functions do not partially overlap.
  • The url can be: an empty string, a relative path (usually indicates an internal node module), an absolute file path (usually for CJS), a file URL (file://, for ESM), or some other protocol.
  • The same ScriptCov can have multiple functions with the same functionName.


Coverage data for a single function. This is the main unit of coverage: it has a list of ranges forming a tree covering the AST and holding count data. A function coverage can correspond to a function declaration or expression, an arrow function, or the whole source text (anything outside of a function).

interface FunctionCov {
  functionName: string;
  ranges: RangeCov[];
  isBlockCoverage: boolean;
pub struct FunctionCov {
  pub function_name: String,
  pub ranges: Vec<RangeCocv>,
  pub is_block_coverage: bool,


  • ranges is always non-empty. The first range is called the "root range".
  • isBlockCoverage indicates if the function has block coverage information.
    • false means that there is a single range and its count is the number of times the function was called. It does not say anything about what happens inside the body of the function, some blocks may be skipped or repeated but this is not measured when isBlockCoverage is false. There's a single range spanning the whole function and its count is just number of calls to this function.
    • true means that the ranges form a tree of blocks representing how many times each statement or expression inside was executed. It detects skipped or repeated statements. The root range counts the number of function calls.
  • The functionName can be an empty string. This is common for the FunctionCov representing the whole module.
  • Inside a single FunctionCov, the ranges form a tree based on their inclusion relation.
  • If you get matching FunctionCov values from different ProcessCov, the ranges may partially overlap (their number and offsets can vary per process).
  • Inside a ScriptCov, a FunctionCov is uniquely identified by the span of its root range.


  • A ScriptCov cannot contain two FunctionCov with the same span but different isBlockCoverage values. This can happen across different ProcessCov: a FunctionCov with isBlockCoverage: true in one ProcessCov and a FunctionCov with the same span but with isBlockCoverage: false in another ProcessCov.


Range coverage is the primary unit of information: it has a count for the number of times a span of code was executed. The offsets and counts are absolute values (relative to the script). The count for an AST node is given by the smallest range containing this node.

interface RangeCov {
  startOffset: number;
  endOffset: number;
  count: number;
pub struct FunctionCov {
  pub start_offset: usize,
  pub end_offset: usize,
  pub count: i64,


  • count >= 0
  • startOffset >= 0
  • startOffset < endOffset


Helpers to manipulate V8 coverage files






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