For instance, I have the following code:

if ("a" !== "a") {
    console.log('really?');
}

var a = 5;

Then I write uglifyjs code.js -o code.min.js. As a result, I have the following:

if("a"!=="a"){console.log("really?")}var a=5;

How do I enable removing the dead code inside the if-statement?

For instance, I have the following code:

if ("a" !== "a") {
    console.log('really?');
}

var a = 5;

Then I write uglifyjs code.js -o code.min.js. As a result, I have the following:

if("a"!=="a"){console.log("really?")}var a=5;

How do I enable removing the dead code inside the if-statement?

• javascript
• uglifyjs

• What's the dead code? console.log? or the entire if block? The former is a perfectly valid code with perfectly valid purpose. It is very different than optimizingif(a == 1 || a == 1). Plus I think uglifier does very limited optimization. – CppLearner Commented Aug 20, 2015 at 4:48
• The code inside the if-block will never run. So, it's dead. – user2991036 Commented Aug 20, 2015 at 4:49
• you need static analysis tool, not just AST parser. – CppLearner Commented Aug 20, 2015 at 4:51
• 1 @CppLearner, you surely need to know what you're disputing about. Stackoverflow is not the place where you express your thoughts or doubts and especially not the place where you convince others that you know what you actually don't know. UglifyJS2 does dead code removal just perfectly. – meandre Commented Nov 12, 2015 at 9:08

2 Answers 2

Despite this question has already got an accepted answer, I think it's worth mentioning that

1. UglifyJS2 does remove dead code

2. To turn this feature on, you need to set appropriate option either in CLI (uglifyjs --compress unused,dead_code) or in the options object if you invoke uglify programmatically (uglify(compress: { unused: true, dead_code: true });).

As per the README for uglifyjs, the maintainer has shifted development effort to UglifyJS2. The README also says that it only removes:

some unreachable code and warn about it (code that follows a return, throw, break or continue statement, except function/variable declarations).

Uglify2 does a more comprehensive job. I tested your code on the demo site and it does indeed remove the whole if statement. It also supports 'conditional compilation' (or maybe more correctly conditional code removal) by allowing you to define globals at the command line when uglifying.