UglifyJS — the name mangler

UglifyJS can reduce names of local variables and functions usually to single-letters. That's a safe optimization provided the following stand:

• You don't directly call the global eval function.
• You don't use the with statement.
• You don't access the Function.name property.

UglifyJS can detect the first two cases and it will disable the mangler automatically in code areas that are poisoned by usage of eval or with. UglifyJS cannot detect if you're using the function's name property, however; in such code the mangler can potentially break your code. For example the JADE template engine is affected by this issue (coupled with its author “I am the smartest and know what's best” attitude).

SYNOPSIS

ast = UglifyJS.parse(code);
ast.figure_out_scope();
ast.compute_char_frequency();
ast.mangle_names();
code = ast.print_to_string();

That's all there is to it. Make sure to call figure_out_scope() first, then call mangle_names() on the toplevel node. Optionally insert the compute_char_frequency() there to get names that are optimized for GZip compression (names will be generated using the most frequent characters first).

If you also compress your code, then you want to mangle names after compression, since the compressor might drop unused identifiers, unreachable code etc. The full sequence would be this:

ast = UglifyJS.parse(code);

// compressor needs figure_out_scope too
ast.figure_out_scope();
compressor = UglifyJS.Compressor()
ast = ast.transform(compressor);

// need to figure out scope again so mangler works optimally
ast.figure_out_scope();
ast.compute_char_frequency();
ast.mangle_names();

// get Ugly code back :)
code = ast.print_to_string();