unknown directives retained

[kzc]

Bug report or feature request?

feature request

Uglify version (uglifyjs -V)

uglify-js 3.3.28

JavaScript input

$ bin/uglifyjs -V
uglify-js 3.3.28

$ echo '"use strict";"A";function f(){"use strict";"B"}f();f();' | bin/uglifyjs -c toplevel
"use strict";"A";function f(){"B"}f(),f();

An older uglify version:

$ uglifyjs -V
uglify-js 2.7.5

$ echo '"use strict";"A";function f(){"use strict";"B"}f();f();' | uglifyjs -c
WARN: Dropping side-effect-free statement [-:1,43]
"use strict";"A";function f(){}f(),f();

Is it worth adding a new "directives" compress options defaulting to true to drop directives other than "use strict" and "use asm"? There might be obscure use cases with custom directives out in the wild. Perhaps this option could default to allowable directives - i.e.: -c directives=["use strict", "use asm"].

[kzc]

It appears that older versions of uglify also retained the unknown directives within functions - the string "B" above did not have a trailing semicolon, so it was not considered a directive.

$ uglifyjs -V
uglify-js 2.7.5

$ echo '"use strict";"A";function f(){"use strict";"B";}f();f();' | uglifyjs -c passes=9
"use strict";"A";function f(){"B"}f(),f();

Whether unknown directives were retained historically is not important. The question is how should it work in the Compressor.

[alexlamsl]

A new directives flag which retains only "use asm" and "use strict" (case sensitive) sounds fine to me - PR is welcome.

[kzc]

Was first noticed while debugging some ESTree AST. Must not be a real issue in the wild. Both babel-minify and Google Closure retain the unknown directives - albeit with a warning for Closure. Maybe they are trying to be future compatible.

If I add such a compress option to terser I'll post a link here.