Function Coupling — tsmetrics Docs

What it measures

Function Coupling measures the fan-out of a module — the number of distinct import sources it depends on. A high fan-out means the module has many external dependencies, making it harder to change, test in isolation, and understand.

tsmetrics classifies imports into three categories:

Relative — paths starting with ./ or ../ (intra-project)
Package — npm packages, Node built-ins, or path aliases
Type-only — import type { … } imports (erased at runtime)

How it's computed

Fan-Out = |distinct import sources| Multiple imports from the same source count as 1. Dynamic imports and re-exports are not counted. If a source appears in both value and type-only imports, it is classified as a value (runtime) dependency.

Example

TypeScript — fan-out: 4

import { useState } from 'react';           // package
import { db } from './database';             // relative
import { logger } from './logger';           // relative
import { db } from './database';             // duplicate → still 1
import type { User } from './types';         // type-only
// distinct sources: react, ./database, ./logger, ./types = 4

Tip: A fan-out above 10 is a warning sign. Consider whether the module is doing too much, or whether some imports could be consolidated via a shared utility layer.

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