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**Description:** This is learnt from https://github.com/terser/terser/pull/1044 **Key point:** all the cases (statements) after the last side-effect and non-terminated one are useless. There are also some points on whether a statement is side-effect free: - **Var declaration:** I think var declaration always contains side effect. - **Function declaration:** In non-strict mode, function declaration is same as var declaration. In strict mode, function is declared in enclosing block scope, so it's side-effect free. But there is a bad case when we call the function before it is declared in the switch case: ```js "use strict"; const k = (() => { let x = 1; switch (x) { case y(): case x: { async function y() { console.log(123); } } } return x; })(); ``` **This is an existing bug.** I'm not sure whether we should fix it since it is a very bad code and I also find some similar bad cases with terser. ~I'm also not sure it's appropriate to introduce context variable `in_strict` for `StmtExt:: may_have_side_effects`, because `in_strict` could change from `false` to `true`. But this is a **conservative** mistake and does not break the program. Maybe we should use visitor for context-aware side effect checker in the future.~ **Related issue:** - Closes https://github.com/swc-project/swc/issues/8953 |
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