compress: unreachable switch case removal drops a CaseBlock function initializer

Author: ko-successCreated Sep 1, 2026Updated Sep 1, 2026

Version and config

terser 5.51.2

javascript

{
  module: false,
  compress: true,
  mangle: {},
  output: {},
  parse: {},
  rename: {}
}

Input program

javascript

switch (0) {
  case 0: console.log(g()); break; // fn
  case 1: function g() { return "fn"; }
}

terser output

javascript

var g; console.log(g()); // TypeError: g is not a function

Expected result

The output should preserve the CaseBlock function binding, or perform an equivalent optimization such as:

javascript

console.log("fn"); // fn

A FunctionDeclaration in any case clause is part of the lexical declarations of the whole CaseBlock.

When the switch statement is evaluated, BlockDeclarationInstantiation initializes the function binding before the selected case is evaluated. Therefore, although the statements of case 1 are unreachable, removing its function declaration also removes observable declaration initialization.

Specification references:

Source: terser/terser

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