jest-diff: Export as ECMAScript module

[pedrottimark]

Summary

Because a caller might need to construct Diff instances to write a custom cleanup algorithm, it must be exported as a class value instead of a type.

Breaking change: Because we rewrote as ECMAScript and TypeScript exports, then the class value can be used in type declarations! Thank you, Simen for suggesting this solution ❤️

CommonJS callers rewrite default export:

const diff = require('jest-diff'); // Jest 24 or earlier
const diff = require('jest-diff').default; // Jest 25 or later

But see README.md because new named export diffStringsUnified might be better for you

However, if I understand the situation correctly, for TypeScript declarations like diff: Diff or diffs: Array<Diff> when the class is imported across a package boundary, an additional type declaration is needed to avoid error TS2749: 'Diff' refers to a value, but is being used as a type here.

Corrected and edited README.md

Did not add to CHANGELOG.md because it is an immediate Began as correction to #8841

Test plan

• Added diffStringsRaw.test.ts which verifies exports, including Diff as type
• The use in jest-matcher-utils also verifies exports, including Diff as type

[codecov-io]

Codecov Report

Merging #8873 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #8873      +/-   ##
==========================================
- Coverage   64.12%   64.11%   -0.01%     
==========================================
  Files         275      275              
  Lines       11629    11623       -6     
  Branches     2845     2845              
==========================================
- Hits         7457     7452       -5     
  Misses       3545     3545              
+ Partials      627      626       -1

Impacted Files	Coverage Δ
packages/jest-diff/src/index.ts	78.04% <ø> (-2.39%)	⬇️
packages/jest-snapshot/src/print.ts	70.37% <100%> (ø)	⬆️
packages/jest-matcher-utils/src/index.ts	82.35% <100%> (-0.12%)	⬇️
packages/jest-diff/src/printDiffs.ts	95% <0%> (+1.66%)	⬆️

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[pedrottimark]

Do y’all think it would help or hurt to distinguish versions for breaking change in README:

• import diffLinesUnified from 'jest-diff'; in ECMAScript modules
• const diffLinesUnified = require('jest-diff').default; in CommonJS modules (jest-diff version 25 or later)
• const diffLinesUnified = require('jest-diff'); in CommonJS modules (jest-diff version 24 or earlier)

And also move ECMAScript example above CommonJS example?

And identify version of new options and named exports?

[SimenB]

The readme won't be included for jest 24, so I don't think that's needed.

[chrisblossom]

Wouldn't this be better as a named export instead of default?

const diff = require('jest-diff'); // Jest 24 or earlier
const { diff } = require('jest-diff'); // Jest 25 or later

[SimenB]

Either works for me, I'll leave it to @pedrottimark 🙂 It does make it cleaner for CJS, though

[chrisblossom]

I bring this up because it probably should be standardized across all of jest’s packages. My vote goes to named exports for whatever its worth.

[pedrottimark]

Thank you for bringing this up.

Among others, Nicholas C. Zakas has explained benefits of named versus default export.

The only reason why it might not happen before Jest 25 is that I am right now exploring how the public API might grow so a naming convention for the exports will be consistent.

[github-actions]

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