Replace-Switches: Rewrites Switch Statements in JavaScript Code

This JS library makes translation of valid JavaScript code given as input in the following way: the library converts all switch statements present in the piece of code to loop and if statements. So, put simply, this library allows one to remove all switches from a JavaScript piece of code, while keeping the semantics of the code.

At the first glance, one can think that the solution should be easy like e.g. turning a switch statement into a simple if-elif-else statement. But this is not true if we remember the problems listed below. The thorough suite of unit-tests is present in the repository and checks a really vast number of testing cases. All the problems listed below have corresponding unit-tests in the suite:

• There can be no break statement in a case branch, so after execution of the case branch we can fall through to the next case branch:

  switch(1) { case 1: // execute this; case 2: // and this; default: // and this }

• The number of times of evaluation of a discriminant of a switch and of test expressions of case branches of a switch should be preserved. This is because the discriminant and the test expressions can be function calls with side effects:

  switch(function_with_side_effects1()) { case function_with_side_effects2(): // do stuff }

• The default case is not necessarily the last case of a switch and also may have no break statement:

  function f() { console.log("function f."); return 1; }
  function g() { console.log("function g."); return 2; }
  function h() { console.log("function h."); return 3; }
  
  switch(h()){ case f(): console.log("case 1."); 
               default:  console.log("default."); 
               case g(): console.log("case 2."); }
  
  // Output: function h. function f. function g. default. case 2.
  

• There can be nested switch statements inside a case branch of a switch statement:

  switch(...) { case 1: switch (...) { ... } }

• There can be nested switch statements in a function definition present in a switch’s discriminant:

  switch(f(){switch(...){...}}) {...}

• There can be labeled break statements in a case branch of a switch statement.

• A break statement present in a case branch of a switch should not be necessarily the break statement associated with the case branch:

  switch(...) { case 1: for(;;){break} }

• A break statement of a case branch can be inside a block statement or several block statements:

  switch(...) { case 1: }

• Many other problems which I forgot to mention. The difficulty of the research problem is also partly explained in the documentation file docs/switch_problem.pdf present in the repository.

Dependencies

JavaScript libraries esprima and escodegen are already present in the repository.

Usage

Usage of the library is extremely simple and can be seen in the file with the unit-tests. But the idea is present below:

const switchRemoval = require("./remove-switches");
var newSourceString = switchRemoval.removeSwitches(oldSourceString);

Example

The input of the unit-test #76 is the following:

function case1() {
    console.log("function case1() called");
    return 1;
}

function case2() {
    console.log("function case2() called");
    return 2;
}

switch (case1()) {
    case case1():
                console.log("case1");
    default:
                console.log("default");
    case case2():
                console.log("case2");
                break;
}console.log("end");

The resulting piece of code is the following:

function case1() {
    console.log('function case1() called');
    return 1;
}
function case2() {
    console.log('function case2() called');
    return 2;
}
do {
    const _uhbhl9pob = case1();
    if (_uhbhl9pob === case1()) {
        console.log('case1');
        console.log('default');
        console.log('case2');
        break;
        break;
    }
    if (_uhbhl9pob === case2()) {
        console.log('case2');
        break;
        break;
    }
    console.log('default');
    console.log('case2');
    break;
    break;
} while (false);
console.log('end');

The output of both the pieces of code is:

case1
default
case2
end