Question

I want to use Closure Compiler to minify/compress JS code.

the problem is that it doesn’t minify as well as I expect it to. consider the code below. when I pass the string

var func = function ( someArgument ) {
  alert ( someArgument ); 
  return someArgument; 
}

I expect the minified code to rename “someArgument” to something much shorter, like “a”.

is that the way it is or am I doing something wrong? TIA

public static void Compress( String src ) {

    ByteArrayOutputStream err = new ByteArrayOutputStream();

    CompilerOptions opt = new CompilerOptions();

    CompilationLevel.ADVANCED_OPTIMIZATIONS.setDebugOptionsForCompilationLevel( opt );

    Compiler.setLoggingLevel( Level.OFF );
    Compiler compiler = new Compiler( new PrintStream( err ) );
    compiler.disableThreads();

    List<SourceFile> externs = Collections.emptyList();
    List<SourceFile> inputs = Arrays.asList( SourceFile.fromCode( "javascript-code.js", src) );

    Result result = compiler.compile( externs, inputs, opt );

    System.out.println( "source: " + compiler.toSource() );
}

Answer 1

You are using setDebugOptionsForCompilationLevel(), you want setOptionsForCompilationLevel(). From the Source this is what setDebugOptionsForCompilationLevel is doing:

public void setDebugOptionsForCompilationLevel(CompilerOptions options) {
    options.anonymousFunctionNaming = AnonymousFunctionNamingPolicy.UNMAPPED;
    options.generatePseudoNames = true;
    options.removeClosureAsserts = false;
    // Don't shadow variables as it is too confusing.
    options.shadowVariables = false;
}

While this is what setOptionsForCompilationLevel() is doing:

// All the safe optimizations.
options.dependencyOptions.setDependencySorting(true);
options.closurePass = true;
options.foldConstants = true;
options.coalesceVariableNames = true;
options.deadAssignmentElimination = true;
options.extractPrototypeMemberDeclarations = true;
options.collapseVariableDeclarations = true;
options.convertToDottedProperties = true;
options.rewriteFunctionExpressions = true;
options.labelRenaming = true;
options.removeDeadCode = true;
options.optimizeArgumentsArray = true;
options.collapseObjectLiterals = true;
options.protectHiddenSideEffects = true;

// All the advance optimizations.
options.removeClosureAsserts = true;
options.aliasKeywords = true;
options.reserveRawExports = true;
options.setRenamingPolicy(
    VariableRenamingPolicy.ALL, PropertyRenamingPolicy.ALL_UNQUOTED);
options.shadowVariables = true;
options.removeUnusedPrototypeProperties = true;
options.removeUnusedPrototypePropertiesInExterns = true;
options.collapseAnonymousFunctions = true;
options.collapseProperties = true;
options.checkGlobalThisLevel = CheckLevel.WARNING;
options.rewriteFunctionExpressions = true;
options.smartNameRemoval = true;
options.inlineConstantVars = true;
options.setInlineFunctions(Reach.ALL);
options.inlineGetters = true;
options.setInlineVariables(Reach.ALL);
options.flowSensitiveInlineVariables = true;
options.computeFunctionSideEffects = true;

// Remove unused vars also removes unused functions.
options.setRemoveUnusedVariables(Reach.ALL);

// Move code around based on the defined modules.
options.crossModuleCodeMotion = true;
options.crossModuleMethodMotion = true;

// Call optimizations
options.devirtualizePrototypeMethods = true;
options.optimizeParameters = true;
options.optimizeReturns = true;
options.optimizeCalls = true;

Technically, SIMPLE_OPTIMIZATIONS would give you argument renaming., in case advanced start causing problems with your code (again from the source):

/**
 * SIMPLE_OPTIMIZATIONS performs transformations to the input JS that do not
 * require any changes to JS that depend on the input JS. For example,
 * function arguments are renamed (which should not matter to code that
 * depends on the input JS), but functions themselves are not renamed (which
 * would otherwise require external code to change to use the renamed function
 * names).
 */