feat(javascript): merge consecutive ifs returning or throwing the same value in the minifier - #22507

Summary

A step from oxc's peephole/esbuild.rs and minimize_if_statement.rs groups. terser folds consecutive ifs into one condition only when they close a list it can turn into an expression. When a loop or another statement follows, it keeps them apart:

// function f(a, b) { if (a == null) return true; if (g(a, b)) return true; for (;;) k(); }
// before
function f(a,b){if(null==a)return!0;if(g(a,b))return!0;for(;;)k()}
// after
function f(a,b){if(null==a||g(a,b))return!0;for(;;)k()}

Before terser tightens a statement list, the improve phase now merges each run of if (a) return x; if (b) return x; into if (a || b) return x;, and throw alike. This applies where neither has an else and both leave with the same value. The second test runs only where the first did not leave, as || has it. A bare return;, break or continue is left to terser, which already folds those into the condition more tightly.

Minified against main:

typescript.js and terser.js gain the most. The check is one pass over each statement list terser already walks, and CPU time over the 12 bundles stays within run-to-run noise. The corpora print what they should:

• 6 sources retire from oxcSmaller.js;
• 5 sources are 4–68 bytes shorter but compress 1–7 bytes worse with gzip, because the if they no longer write repeated one gzip matched. They are listed in IMPROVED_YET_BIGGER.

What kind of change does this PR introduce?

feat

Did you add tests for your changes?

Yes, in test/unitCases/syntax-printer.unittest.js:

• An improved case: consecutive ifs that return or throw the same value, followed by more statements, inside a function and a try.
• A kept case, which stays as terser writes it: consecutive ifs that return different values or jump to different labels.

Does this PR introduce a breaking change?

No.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

docs/syntax.md will list this rewrite with the other minifier changes in one docs update.

Use of AI

Written with Claude Code: it found the pattern by diffing react-dom against swc's output, wrote the change and tests, and ran the corpora, size and CPU checks; I reviewed the result.

🤖 Generated with Claude Code

https://claude.ai/code/session_01ViD41i2KdwwGxrDk7yrjeD

Generated by Claude Code

Summary by CodeRabbit

• Improvements
  • Minification can now combine consecutive if statements when they share an equivalent exit, such as returning or throwing the same value. This can reduce repeated conditions and produce shorter output.
  • The optimization applies within blocks, functions, arrow functions, and class static blocks when improvements are enabled.