[JS IR BE] Add generating JS AST for "when"

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt

@@ -9,10 +9,7 @@ import org.jetbrains.kotlin.ir.backend.js.utils.isPrimary
 import org.jetbrains.kotlin.ir.backend.js.utils.parameterCount
 import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
-import org.jetbrains.kotlin.ir.expressions.IrCall
+import org.jetbrains.kotlin.ir.expressions.*
-import org.jetbrains.kotlin.ir.expressions.IrConst
-import org.jetbrains.kotlin.ir.expressions.IrConstKind
-import org.jetbrains.kotlin.ir.expressions.IrExpressionBody
 import org.jetbrains.kotlin.ir.symbols.IrConstructorSymbol
 import org.jetbrains.kotlin.js.backend.ast.*
 
@@ -73,4 +70,9 @@ class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsEx
 
         return JsInvocation(ref, extensionReceiver?.let { listOf(extensionReceiver) + arguments } ?: arguments)
     }
+
+    override fun visitWhen(expression: IrWhen, data: Nothing?): JsExpression {
+        // TODO check when w/o else branch and empty when
+        return expression.toJsNode(this, data, ::JsConditional)!!
+    }
 }

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsStatementTransformer.kt

@@ -37,4 +37,8 @@ class IrElementToJsStatementTransformer : BaseIrElementToJsNodeTransformer<JsSta
     override fun visitVariable(declaration: IrVariable, data: Nothing?): JsStatement {
         return jsVar(declaration.name, declaration.initializer)
     }
+
+    override fun visitWhen(expression: IrWhen, data: Nothing?): JsStatement {
+        return expression.toJsNode(this, data, ::JsIf) ?: JsEmpty
+    }
 }

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/jsAstUtils.kt

@@ -5,8 +5,13 @@
 
 package org.jetbrains.kotlin.ir.backend.js.transformers.irToJs
 
+import org.jetbrains.kotlin.ir.expressions.IrBranch
+import org.jetbrains.kotlin.ir.expressions.IrElseBranch
 import org.jetbrains.kotlin.ir.expressions.IrExpression
+import org.jetbrains.kotlin.ir.expressions.IrWhen
 import org.jetbrains.kotlin.js.backend.ast.JsDynamicScope
+import org.jetbrains.kotlin.js.backend.ast.JsExpression
+import org.jetbrains.kotlin.js.backend.ast.JsNode
 import org.jetbrains.kotlin.js.backend.ast.JsVars
 import org.jetbrains.kotlin.name.Name
 
@@ -21,3 +26,13 @@ fun jsVar(name: Name, initializer: IrExpression?): JsVars {
     val jsInitializer = initializer?.accept(IrElementToJsExpressionTransformer(), null)
     return JsVars(JsVars.JsVar(name.toJsName(), jsInitializer))
 }
+
+fun <T : JsNode, D : Nothing?> IrWhen.toJsNode(tr: BaseIrElementToJsNodeTransformer<T, D>, data: D, node: (JsExpression, T, T?) -> T): T? =
+    branches.foldRight<IrBranch, T?>(null) { br, n ->
+        val body = br.result.accept(tr, data)
+        if (br is IrElseBranch) body
+        else {
+            val condition = br.condition.accept(IrElementToJsExpressionTransformer(), data)
+            node(condition, body, n)
+        }
+    }