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A spy library for TypeScript.

💿 Installation

Use npm or yarn.

npm install --save-dev @mysticatea/spy


  • Node.js 6.0.0 and over.
  • TypeScript 3.1 and over (if you use spy in TypeScript).

📖 Usage

import { spy } from "@mysticatea/spy"

// Create a spy function.
const func1 = spy()

// Call it.

// Check it.
console.log(func1.calls.length) //→ 1
console.log(func1.calls[0]) //→ { type: "return", this: undefined, arguments: [], return: undefined }
console.log(func1.returnedCalls.length) //→ 1
console.log(func1.returnedCalls[0]) //→ { type: "return", this: undefined, arguments: [], return: undefined }
console.log(func1.thrownCalls.length) //→ 0

// Create a spy function with that behavior.
const func2 = spy((value: number): string => {
    if (value < 0) {
        throw new Error("oops!")
    return String(value)
// func2 is `(value: number) => string` and some additional properties.

// Call it.
const retv2 = func2(777)
try { func2(-1) } catch { /*ignore*/ }

// Check it.
console.log(retv2) //→ "777"
console.log(func2.calls.length) //→ 2
console.log(func2.calls[0]) //→ { type: "return", this: undefined, arguments: [1], return: "1" }
console.log(func2.calls[1]) //→ { type: "throw", this: undefined, arguments: [-1], throw: [object Error] }
console.log(func2.returnedCalls.length) //→ 1
console.log(func2.returnedCalls[0]) //→ { type: "return", this: undefined, arguments: [1], return: "1" }
console.log(func2.thrownCalls.length) //→ 1
console.log(func2.thrownCalls[0]) //→ { type: "throw", this: undefined, arguments: [-1], throw: [object Error] }

📚 API Reference

s = spy(func?)


Name Type Description
T * extends (...args: any[]) => any The type of func. If func is omitted, this is () => void.
func T or undefined Optional behavior of the spy.

Return value:

Type Description
Spy<T> The created spy function.

The length property of the spy function can be different to func.length.


Create a spy function.

The spy function will record the call information of itself. You can access the information with calls property and something like.

See below for spy's properties.


The array of call information.

The element of this array has the following properties:

Name Type Description
type string The result type of this call. If the behavior of this spy has thrown an error, this is "throw". Otherwise, this is "return".
this This<T> The this value of this call.
arguments Parameters<T> The arguments of this call.
return ReturnType<T> The return value of this call. This exists only if type === "return".
throw any The thrown value of this call. This exists only if type === "throw".


This is equivalent to s.calls[0].


This is equivalent to s.calls[s.calls.length - 1].


The array of call information, only the call which returned a value successfully.

This is equivalent to s.calls.filter(c => c.type === "return").


This is equivalent to s.returnedCalls[0].


This is equivalent to s.returnedCalls[s.returnedCalls.length - 1].


The array of call information, only the call which threw an error.

This is equivalent to s.calls.filter(c => c.type === "throw").


This is equivalent to s.thrownCalls[0].


This is equivalent to s.thrownCalls[s.thrownCalls.length - 1].


Clear the s.calls in order to reuse this spy.

⚠️ Known Issues

TypeScript cannot infer the this type of methods

const box = {
    value: 0,
    set(value: number): void {
        this.value = value

const f = spy(box.set)
f.calls[0].this //→ inferred to `{}` rather than `typeof box`!

// Workaround:
const f1 = spy<(this: typeof box, value: number) => void>(box.set)
f.calls[0].this //→ correct `typeof box`.

📰 Changelog

❤️ Contributing

Welcome contributing!

Please use GitHub's Issues/PRs.

Development Tools

  • npm test runs tests and measures coverage.
  • npm run build compiles TypeScript source code into dist directory.
  • npm run clean removes the temporary files which are created by npm test and npm run build.
  • npm run lint runs ESLint.
  • npm run watch runs tests with --watch option.


npm i @mysticatea/spy

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