diff --git a/src/compiler/irgen.js b/src/compiler/irgen.js
index 7a059a5b..fcf31b88 100644
--- a/src/compiler/irgen.js
+++ b/src/compiler/irgen.js
@@ -1145,7 +1145,7 @@ class ScriptTreeGenerator {
                 const blockInfo = this.getBlockInfo(block.opcode);
                 if (blockInfo) {
                     const type = blockInfo.info.blockType;
-                    if (type === BlockType.COMMAND) {
+                    if (type === BlockType.COMMAND || type === BlockType.CONDITIONAL) {
                         return this.descendCompatLayer(block);
                     }
                 }
@@ -1386,19 +1386,37 @@ class ScriptTreeGenerator {
      */
     descendCompatLayer (block) {
         this.script.yields = true;
+
         const inputs = {};
-        const fields = {};
         for (const name of Object.keys(block.inputs)) {
-            inputs[name] = this.descendInputOfBlock(block, name);
+            if (!name.startsWith('SUBSTACK')) {
+                inputs[name] = this.descendInputOfBlock(block, name);
+            }
         }
+
+        const fields = {};
         for (const name of Object.keys(block.fields)) {
             fields[name] = block.fields[name].value;
         }
+
+        const blockInfo = this.getBlockInfo(block.opcode);
+        const blockType = (blockInfo && blockInfo.info && blockInfo.info.blockType) || BlockType.COMMAND;
+        const substacks = [];
+        if (blockType === BlockType.LOOP || blockType === BlockType.CONDITIONAL) {
+            const branchCount = blockInfo.info.branchCount;
+            for (let i = 0; i < branchCount; i++) {
+                const inputName = i === 0 ? 'SUBSTACK' : `SUBSTACK${i + 1}`;
+                substacks.push(this.descendSubstack(block, inputName));
+            }
+        }
+
         return {
             kind: 'compat',
             opcode: block.opcode,
+            blockType,
             inputs,
-            fields
+            fields,
+            substacks
         };
     }
 
diff --git a/src/compiler/jsgen.js b/src/compiler/jsgen.js
index 32a0d1d7..ce8df777 100644
--- a/src/compiler/jsgen.js
+++ b/src/compiler/jsgen.js
@@ -1,5 +1,6 @@
 const log = require('../util/log');
 const Cast = require('../util/cast');
+const BlockType = require('../extension-support/block-type');
 const VariablePool = require('./variable-pool');
 const jsexecute = require('./jsexecute');
 const environment = require('./environment');
@@ -760,7 +761,21 @@ class JSGenerator {
             // If the last command in a loop returns a promise, immediately continue to the next iteration.
             // If you don't do this, the loop effectively yields twice per iteration and will run at half-speed.
             const isLastInLoop = this.isLastBlockInLoop();
-            this.source += `${this.generateCompatibilityLayerCall(node, isLastInLoop)};\n`;
+
+            const blockType = node.blockType;
+            if (blockType === BlockType.COMMAND || blockType === BlockType.HAT) {
+                this.source += `${this.generateCompatibilityLayerCall(node, isLastInLoop)};\n`;
+            } else if (blockType === BlockType.CONDITIONAL) {
+                this.source += `switch (Math.round(${this.generateCompatibilityLayerCall(node, isLastInLoop)})) {\n`;
+                for (let i = 0; i < node.substacks.length; i++) {
+                    this.source += `case ${i + 1}: {\n`;
+                    this.descendStack(node.substacks[i], new Frame(false));
+                    this.source += `break;\n`;
+                    this.source += `}\n`;
+                }
+                this.source += `}\n`;
+            }
+
             if (isLastInLoop) {
                 this.source += 'if (hasResumedFromPromise) {hasResumedFromPromise = false;continue;}\n';
             }
diff --git a/test/fixtures/tw-conditional.sb3 b/test/fixtures/tw-conditional.sb3
new file mode 100644
index 00000000..fc72114c
Binary files /dev/null and b/test/fixtures/tw-conditional.sb3 differ
diff --git a/test/integration/tw_custom_conditional_and_loop.js b/test/integration/tw_custom_conditional_and_loop.js
new file mode 100644
index 00000000..6d90c245
--- /dev/null
+++ b/test/integration/tw_custom_conditional_and_loop.js
@@ -0,0 +1,90 @@
+const fs = require('fs');
+const path = require('path');
+const {test} = require('tap');
+const VirtualMachine = require('../../src/virtual-machine');
+const Scratch = require('../../src/extension-support/tw-extension-api-common');
+
+class LoopsAndThings {
+    getInfo () {
+        return {
+            id: 'loopsAndThings',
+            name: 'Loops and things test',
+            blocks: [
+                {
+                    opcode: 'conditional',
+                    blockType: Scratch.BlockType.CONDITIONAL,
+                    text: 'run branch [BRANCH] of',
+                    arguments: {
+                        BRANCH: {
+                            type: Scratch.ArgumentType.NUMBER,
+                            defaultValue: 1
+                        }
+                    },
+                    branchCount: 3
+                },
+                {
+                    opcode: 'loop',
+                    blockType: Scratch.BlockType.LOOP,
+                    text: 'my repeat [TIMES]',
+                    arguments: {
+                        TIMES: {
+                            type: Scratch.ArgumentType.NUMBER,
+                            defaultValue: 10
+                        }
+                    }
+                }
+            ]
+        };
+    }
+
+    conditional ({BRANCH}, util) {
+        return Scratch.Cast.toNumber(BRANCH);
+    }
+
+    loop ({TIMES}, util) {
+        const times = Math.round(Scratch.Cast.toNumber(TIMES));
+        if (typeof util.stackFrame.loopCounter === 'undefined') {
+            util.stackFrame.loopCounter = times;
+        }
+        util.stackFrame.loopCounter--;
+        if (util.stackFrame.loopCounter >= 0) {
+            return true;
+        }
+    }
+}
+
+const compilerAndInterpreter = (name, callback) => {
+    test(`${name} - interpreted`, t => {
+        callback(t, {
+            enabled: false
+        });
+    });
+    test(`${name} - compiled`, t => {
+        callback(t, {
+            enabled: true
+        });
+    });
+};
+
+compilerAndInterpreter('CONDITIONAL', (t, co) => {
+    t.plan(1);
+
+    const vm = new VirtualMachine();
+    vm.setCompilerOptions(co);
+    vm.extensionManager.addBuiltinExtension('loopsAndThings', LoopsAndThings);
+    
+    vm.loadProject(fs.readFileSync(path.join(__dirname, '../fixtures/tw-conditional.sb3'))).then(() => {
+        let okayCount = 0;
+        vm.runtime.on('SAY', (target, type, text) => {
+            if (text === 'OK!') {
+                okayCount++;
+            } else if (text === 'end') {
+                t.equal(okayCount, 5);
+                t.end();
+            }
+        });
+
+        vm.greenFlag();
+        vm.runtime._step();
+    });
+});