Add uvu/spy module

[marvinhagemeister]

This PR adds a new uvu/spy package that exports a function to spy on function calls. This works with both sync and async functions. All function arguments, return values and the proper this context is captured and can be asserted against.

Example:

import { test } from 'uvu';
import * as assert from 'uvu/assert';
import { spy } from 'uvu/spy';

test('check if function is called twice', () => {
  const testFunction = spy((a, b) => a + b);

  add(1, 2);
  add(3, 4);

  // Check if called twice
  assert.is(testFunction.calls.length, 2);

  // Check return value of 1st call
  assert.is(testFunction.calls[0].return, 3);

  // Check arguments of 2nd call
  assert.equal(testFunction.calls[1].args, [3, 4]);
});

Fixes #10 .

[codecov-commenter]

Codecov Report

Merging #32 into master will increase coverage by 1.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   89.36%   90.47%   +1.11%     
==========================================
  Files           3        4       +1     
  Lines         282      315      +33     
==========================================
+ Hits          252      285      +33     
  Misses         30       30              

Impacted Files	Coverage Δ
src/spy.js	100.00% <100.00%> (ø)
src/assert.js	91.35% <0.00%> (+1.08%)	⬆️
src/index.js	78.09% <0.00%> (+2.56%)	⬆️

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[tjoskar]

Looking good!
Just a side note: there is a similar lib, written in typescript, over here: https://github.com/tjoskar/simple-spy/blob/master/src/index.ts

[marvinhagemeister]

@tjoskar Love these improvements! Spoke with @lukeed in a few weeks ago and he wasn't sure about the approach chosen in this PR and was looking more for something proxy based. So I'm not sure if this PR will get mainlined at all.

[43081j]

fwiw there's also: https://github.com/43081j/hanbi

apologies for the self-promotion but it is rather tiny and more complete than what exists here, if you don't want to maintain your own. there's a few similar libraries around, too.

i'd be happy to do some uvu integration for it if that'd help