diff --git a/runtime/JavaScript/src/antlr4/atn/ATN.js b/runtime/JavaScript/src/antlr4/atn/ATN.js
index b08131d40..78570513b 100644
--- a/runtime/JavaScript/src/antlr4/atn/ATN.js
+++ b/runtime/JavaScript/src/antlr4/atn/ATN.js
@@ -6,91 +6,117 @@
 var LL1Analyzer = require('./../LL1Analyzer').LL1Analyzer;
 var IntervalSet = require('./../IntervalSet').IntervalSet;
 
-function ATN(grammarType , maxTokenType) {
+class ATN {
+    constructor(grammarType , maxTokenType) {
 
-    // Used for runtime deserialization of ATNs from strings///
-    // The type of the ATN.
-    this.grammarType = grammarType;
-    // The maximum value for any symbol recognized by a transition in the ATN.
-    this.maxTokenType = maxTokenType;
-    this.states = [];
-    // Each subrule/rule is a decision point and we must track them so we
-    //  can go back later and build DFA predictors for them.  This includes
-    //  all the rules, subrules, optional blocks, ()+, ()* etc...
-    this.decisionToState = [];
-    // Maps from rule index to starting state number.
-    this.ruleToStartState = [];
-    // Maps from rule index to stop state number.
-    this.ruleToStopState = null;
-    this.modeNameToStartState = {};
-    // For lexer ATNs, this maps the rule index to the resulting token type.
-    // For parser ATNs, this maps the rule index to the generated bypass token
-    // type if the
-    // {@link ATNDeserializationOptions//isGenerateRuleBypassTransitions}
-    // deserialization option was specified; otherwise, this is {@code null}.
-    this.ruleToTokenType = null;
-    // For lexer ATNs, this is an array of {@link LexerAction} objects which may
-    // be referenced by action transitions in the ATN.
-    this.lexerActions = null;
-    this.modeToStartState = [];
-
-    return this;
-}
+        // Used for runtime deserialization of ATNs from strings///
+        // The type of the ATN.
+        this.grammarType = grammarType;
+        // The maximum value for any symbol recognized by a transition in the ATN.
+        this.maxTokenType = maxTokenType;
+        this.states = [];
+        // Each subrule/rule is a decision point and we must track them so we
+        //  can go back later and build DFA predictors for them.  This includes
+        //  all the rules, subrules, optional blocks, ()+, ()* etc...
+        this.decisionToState = [];
+        // Maps from rule index to starting state number.
+        this.ruleToStartState = [];
+        // Maps from rule index to stop state number.
+        this.ruleToStopState = null;
+        this.modeNameToStartState = {};
+        // For lexer ATNs, this maps the rule index to the resulting token type.
+        // For parser ATNs, this maps the rule index to the generated bypass token
+        // type if the
+        // {@link ATNDeserializationOptions//isGenerateRuleBypassTransitions}
+        // deserialization option was specified; otherwise, this is {@code null}.
+        this.ruleToTokenType = null;
+        // For lexer ATNs, this is an array of {@link LexerAction} objects which may
+        // be referenced by action transitions in the ATN.
+        this.lexerActions = null;
+        this.modeToStartState = [];
+    }
 
 // Compute the set of valid tokens that can occur starting in state {@code s}.
 //  If {@code ctx} is null, the set of tokens will not include what can follow
 //  the rule surrounding {@code s}. In other words, the set will be
 //  restricted to tokens reachable staying within {@code s}'s rule.
-ATN.prototype.nextTokensInContext = function(s, ctx) {
-    var anal = new LL1Analyzer(this);
-    return anal.LOOK(s, null, ctx);
-};
+    nextTokensInContext(s, ctx) {
+        var anal = new LL1Analyzer(this);
+        return anal.LOOK(s, null, ctx);
+    }
 
 // Compute the set of valid tokens that can occur starting in {@code s} and
 // staying in same rule. {@link Token//EPSILON} is in set if we reach end of
 // rule.
-ATN.prototype.nextTokensNoContext = function(s) {
-    if (s.nextTokenWithinRule !== null ) {
+    nextTokensNoContext(s) {
+        if (s.nextTokenWithinRule !== null ) {
+            return s.nextTokenWithinRule;
+        }
+        s.nextTokenWithinRule = this.nextTokensInContext(s, null);
+        s.nextTokenWithinRule.readOnly = true;
         return s.nextTokenWithinRule;
     }
-    s.nextTokenWithinRule = this.nextTokensInContext(s, null);
-    s.nextTokenWithinRule.readOnly = true;
-    return s.nextTokenWithinRule;
-};
 
-ATN.prototype.nextTokens = function(s, ctx) {
-    if ( ctx===undefined ) {
-        return this.nextTokensNoContext(s);
-    } else {
-        return this.nextTokensInContext(s, ctx);
+    nextTokens(s, ctx) {
+        if ( ctx===undefined ) {
+            return this.nextTokensNoContext(s);
+        } else {
+            return this.nextTokensInContext(s, ctx);
+        }
     }
-};
 
-ATN.prototype.addState = function( state) {
-    if ( state !== null ) {
-        state.atn = this;
-        state.stateNumber = this.states.length;
+    addState(state) {
+        if ( state !== null ) {
+            state.atn = this;
+            state.stateNumber = this.states.length;
+        }
+        this.states.push(state);
     }
-    this.states.push(state);
-};
 
-ATN.prototype.removeState = function( state) {
-    this.states[state.stateNumber] = null; // just free mem, don't shift states in list
-};
-
-ATN.prototype.defineDecisionState = function( s) {
-    this.decisionToState.push(s);
-    s.decision = this.decisionToState.length-1;
-    return s.decision;
-};
-
-ATN.prototype.getDecisionState = function( decision) {
-    if (this.decisionToState.length===0) {
-        return null;
-    } else {
-        return this.decisionToState[decision];
+    removeState(state) {
+        this.states[state.stateNumber] = null; // just free mem, don't shift states in list
     }
-};
+
+    defineDecisionState(s) {
+        this.decisionToState.push(s);
+        s.decision = this.decisionToState.length-1;
+        return s.decision;
+    }
+
+    getDecisionState(decision) {
+        if (this.decisionToState.length===0) {
+            return null;
+        } else {
+            return this.decisionToState[decision];
+        }
+    }
+
+    getExpectedTokens(stateNumber, ctx ) {
+        if ( stateNumber < 0 || stateNumber >= this.states.length ) {
+            throw("Invalid state number.");
+        }
+        var s = this.states[stateNumber];
+        var following = this.nextTokens(s);
+        if (!following.contains(Token.EPSILON)) {
+            return following;
+        }
+        var expected = new IntervalSet();
+        expected.addSet(following);
+        expected.removeOne(Token.EPSILON);
+        while (ctx !== null && ctx.invokingState >= 0 && following.contains(Token.EPSILON)) {
+            var invokingState = this.states[ctx.invokingState];
+            var rt = invokingState.transitions[0];
+            following = this.nextTokens(rt.followState);
+            expected.addSet(following);
+            expected.removeOne(Token.EPSILON);
+            ctx = ctx.parentCtx;
+        }
+        if (following.contains(Token.EPSILON)) {
+            expected.addOne(Token.EOF);
+        }
+        return expected;
+    }
+}
 
 // Computes the set of input symbols which could follow ATN state number
 // {@code stateNumber} in the specified full {@code context}. This method
@@ -111,32 +137,6 @@ ATN.prototype.getDecisionState = function( decision) {
 // number {@code stateNumber}
 var Token = require('./../Token').Token;
 
-ATN.prototype.getExpectedTokens = function( stateNumber, ctx ) {
-    if ( stateNumber < 0 || stateNumber >= this.states.length ) {
-        throw("Invalid state number.");
-    }
-    var s = this.states[stateNumber];
-    var following = this.nextTokens(s);
-    if (!following.contains(Token.EPSILON)) {
-        return following;
-    }
-    var expected = new IntervalSet();
-    expected.addSet(following);
-    expected.removeOne(Token.EPSILON);
-    while (ctx !== null && ctx.invokingState >= 0 && following.contains(Token.EPSILON)) {
-        var invokingState = this.states[ctx.invokingState];
-        var rt = invokingState.transitions[0];
-        following = this.nextTokens(rt.followState);
-        expected.addSet(following);
-        expected.removeOne(Token.EPSILON);
-        ctx = ctx.parentCtx;
-    }
-    if (following.contains(Token.EPSILON)) {
-        expected.addOne(Token.EOF);
-    }
-    return expected;
-};
-
 ATN.INVALID_ALT_NUMBER = 0;
 
 module.exports = ATN;