Create a Spy Function (Test Double)

intermediate

By FrontendAtlas Editorial · Updated Feb 5, 2026

Implement createSpy(impl?) to produce a callable spy that records every call (args + this) and optionally delegates to an implementation. Spies are a core unit-testing tool to verify behavior at boundaries (network, storage, analytics) without needing a full mocking library.

Arguments

impl: Function (optional) — Optional implementation to call. The spy must preserve the caller's this value.

Returns

Function —

A spy function with metadata: calls, thisValues, callCount(), lastCall(), reset().

Examples

const spy = createSpy((a, b) => a + b);
spy(1, 2); // => 3
spy.calls; // => [[1, 2]]
spy.callCount(); // => 1

const obj = { x: 2, fn: createSpy(function (n) { return this.x + n; }) };
obj.fn(3); // => 5 (this preserved)
obj.fn.thisValues[0] === obj; // true

Solution

Overview

A spy is just a function with extra state. Record calls in arrays, and if an implementation is provided, delegate to it using apply() so this binding is preserved.

Approach: Closure + attached metadata

Steps:

1) Create a function spy(...args).

2) Push args into spy.calls and this into spy.thisValues.

3) If impl exists, return impl.apply(this, args).

4) Attach helpers: callCount(), lastCall(), reset().

Complexity: O(1) per call and O(k) memory for k calls.

export default function createSpy(impl) {
  function spy(...args) {
    spy.calls.push(args);
    spy.thisValues.push(this);
    if (typeof impl === 'function') {
      return impl.apply(this, args);
    }
    return undefined;
  }

  spy.calls = [];
  spy.thisValues = [];
  spy.callCount = () => spy.calls.length;
  spy.lastCall = () => (spy.calls.length ? spy.calls[spy.calls.length - 1] : null);
  spy.reset = () => {
    spy.calls.length = 0;
    spy.thisValues.length = 0;
  };

  return spy;
}

export type SpyFn<TArgs extends any[] = any[], TResult = any> = ((...args: TArgs) => TResult) & {
  calls: TArgs[];
  thisValues: any[];
  callCount: () => number;
  lastCall: () => TArgs | null;
  reset: () => void;
};

export default function createSpy<TArgs extends any[] = any[], TResult = any>(
  impl?: (this: any, ...args: TArgs) => TResult
): SpyFn<TArgs, TResult> {
  const spy = function (this: any, ...args: TArgs) {
    (spy as any).calls.push(args);
    (spy as any).thisValues.push(this);
    if (typeof impl === 'function') {
      return impl.apply(this, args);
    }
    return undefined as any as TResult;
  } as SpyFn<TArgs, TResult>;

  spy.calls = [] as TArgs[];
  spy.thisValues = [] as any[];
  spy.callCount = () => spy.calls.length;
  spy.lastCall = () => (spy.calls.length ? spy.calls[spy.calls.length - 1] : null);
  spy.reset = () => {
    spy.calls.length = 0;
    spy.thisValues.length = 0;
  };

  return spy;
}

Notes & Pitfalls

Pitfalls

Calling impl(...args) loses the caller's this; use impl.apply(this, args).
Forgetting to reset between tests causes order-dependent failures.
Asserting too much about call order can make tests brittle; prefer behavior-based assertions.

Edge cases

No implementation: spy still records calls and returns undefined.
Zero calls: lastCall() should return null.
The spy may be used as a method; thisValues should reflect the receiver.

Techniques

Closure to keep state private.
Attach metadata onto the function object.
Preserve this via apply().

Resources

MDN – Function.prototype.apply

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