Mastering UglifyJS: Advanced While, Conditional, Block, and IF Optimizations

4. While Loop Compression

Rule 4.1 : Remove while loops that will never execute and convert while(true) to for(;;) to save characters.

// while(false) is removed
for(;;){ // while(true) converted, saves 4 characters
  C();
  D();
}

5. Conditional Expression Compression

Rule 5.1 : If a condition is preceded by a logical NOT, drop the NOT and swap the true/false branches. cond ? no() : yes(); Rule 5.2 : When the condition is a constant boolean, replace the whole expression with the selected branch.

yes();
no();

6. Statement Block Compression

Rule 6.1 : Consecutive expression statements can be merged into a single comma expression.

function A(){
  B(), C(), d = 1;
}

Rule 6.2 : Multiple var declarations can be combined into one.

function A(){
  var a, b, c;
}

Rule 6.3 : Statements after return, throw, break or continue that are unreachable can be removed (except declarations that are hoisted).

function A(){
  function B(){ }
  return false;
  var a = 1;
}

Rule 6.4 : Merge the final return with preceding expression statements.

function A(){
  return B(), C(), D();
}

7. IF Branch Optimization

Rule 7.1 : Remove IF/ELSE branches whose condition can be evaluated at compile time.

A();
D();

Rule 7.2 : Eliminate empty IF/ELSE branches by negating the condition.

if (A){
  B();
}
if (!C){
  D();
}

Rule 7.3 : Reverse IF/ELSE when the negated condition yields shorter code.

if (c){
  B();
} else {
  A();
}

Rule 7.4 : Combine nested IFs with an empty ELSE into a single logical AND.

if (A && B){
  C();
}

Rule 7.5 : When the IF block ends with a control‑flow statement, move the ELSE code after the IF.

if (A){
  B();
  return;
}
C();

Rule 7.6 : Merge two single‑statement returns into a conditional expression. return A ? B() : C(); Rule 7.7 : Convert single‑statement IF/ELSE into a ternary expression, then apply the conditional‑expression rules.

Rule 7.8 : When one branch is empty and the other contains a single statement, replace the IF/ELSE with logical AND (&&) or OR (||) expressions.

A && B();
C || D();

For a complete source‑code analysis, refer to the original article linked in the text.