diff --git a/dev/index.php b/dev/index.php
index 6ac56e2..f775650 100644
--- a/dev/index.php
+++ b/dev/index.php
@@ -112,15 +112,20 @@
 				
 				// parse source code
 				/**/const parse_start = performance.now();
-				juicescript.parse(juice_program);
+				let parse_success = juicescript.parse(juice_program);
 				/**/const parse_end = performance.now();
 				/**/juicescript.io.stderr.info("Parsing took " + (parse_end - parse_start) + "ms");
 				
 				// execute program
-				/**/const run_start = performance.now();
-				juicescript.run();
-				/**/const run_end = performance.now();
-				/**/juicescript.io.stderr.info("Running took " + (run_end - run_start) + "ms");
+				if(parse_success){
+					/**/const run_start = performance.now();
+					juicescript.run();
+					/**/const run_end = performance.now();
+					/**/juicescript.io.stderr.info("Running took " + (run_end - run_start) + "ms");
+					
+				} else {
+					juicescript.io.stderr.info("Not executing program due to parse error");
+				}
 			});
 		</script>
 	</head>
diff --git a/src/lexer.js b/src/lexer.js
index 4455aea..785e3ed 100644
--- a/src/lexer.js
+++ b/src/lexer.js
@@ -25,6 +25,10 @@ class Juicescript_lexer {
 		// token list
 		this.token_list = [];
 		
+		// warning and error counter
+		this.warning_count = 0;
+		this.error_count = 0;
+		
 		
 		// SCAN WHOLE SOURCE //
 		while(!this.is_at_end()){
@@ -84,7 +88,7 @@ class Juicescript_lexer {
 				
 			case "=":
 				if		(this.match("="))		this.token_add({type: Juicescript.token_type.EQUAL});
-				else							this.warning("unexpected character '" + char + "'");
+				else							this.error("unexpected character '" + char + "'");
 				break;
 				
 			case "<":
@@ -127,7 +131,7 @@ class Juicescript_lexer {
 				
 				// single slash
 				if(char === "/" && !this.match("/")){
-					this.warning("unexpected character '" + char + "'");
+					this.error("unexpected character '" + char + "'");
 					break;
 				}
 				
@@ -172,8 +176,8 @@ class Juicescript_lexer {
 					break;
 				}
 				
-				// ignore with warning
-				this.warning("unexpected character '" + char + "'");
+				// ignore with error
+				this.error("unexpected character '" + char + "'");
 				break;
 			
 			
@@ -186,8 +190,8 @@ class Juicescript_lexer {
 					break;
 				}
 				
-				// ignore with warning
-				this.warning("unexpected character '" + char + "'");
+				// ignore with error
+				this.error("unexpected character '" + char + "'");
 				break;
 			
 			
@@ -205,8 +209,8 @@ class Juicescript_lexer {
 					break;
 				}
 				
-				// unexpected
-				this.warning("unexpected character '" + char + "'");
+				// unexpected (ignore with error)
+				this.error("unexpected character '" + char + "'");
 				break;
 		}
 	}
@@ -239,8 +243,8 @@ class Juicescript_lexer {
 		
 		// DID WE REACH THE END OF SOURCE WITHOUT TERMINATION? //
 		if(this.is_at_end()){
-			// ignore with warning
-			this.warning("unterminated string");
+			// ignore with error
+			this.error("unterminated string");
 			return;
 		}
 		
@@ -334,8 +338,8 @@ class Juicescript_lexer {
 		
 		// DID WE REACH THE END OF SOURCE WITHOUT TERMINATION? //
 		if(this.is_at_end()){
-			// ignore with warning
-			this.warning("unterminated block comment");
+			// ignore with error
+			this.error("unterminated block comment");
 			return;
 		}
 		
@@ -364,8 +368,8 @@ class Juicescript_lexer {
 				return;
 			}
 			
-			// ignore with warning
-			this.warning("unexpected character '" + this.source.charAt(this.start) + "'");
+			// ignore with error
+			this.error("unexpected character '" + this.source.charAt(this.start) + "'");
 			return;
 		}
 		
@@ -539,8 +543,8 @@ class Juicescript_lexer {
 		
 		// CHECK IF THERE EVEN IS A NAME //
 		if(flag.length <= 0){
-			// ignore with warning
-			this.warning("unexpected character '" + this.source.charAt(this.start) + "'");
+			// ignore with error
+			this.error("unexpected character '" + this.source.charAt(this.start) + "'");
 			return;
 		}
 		
@@ -679,6 +683,11 @@ class Juicescript_lexer {
 		this.io.stderr.info(text, additional);
 	}
 	warning(text, additional){
+		// KEEP TRACK OF PROBLEM //
+		this.warning_count++;
+		
+		
+		// PRINT MESSAGE //
 		// add defaults
 		additional ??= {};
 		additional.line ??= this.line;
@@ -686,12 +695,17 @@ class Juicescript_lexer {
 		// forward
 		this.io.stderr.warning(text, additional);
 	}
-	debug(text, additional){
+	error(text, additional){
+		// KEEP TRACK OF PROBLEM //
+		this.error_count++;
+		
+		
+		// PRINT MESSAGE //
 		// add defaults
 		additional ??= {};
 		additional.line ??= this.line;
 		
 		// forward
-		this.io.stderr.debug(text, additional);
+		this.io.stderr.error(text, additional);
 	}
 }
diff --git a/src/main.js b/src/main.js
index 3c928c2..6bdc7dc 100644
--- a/src/main.js
+++ b/src/main.js
@@ -61,6 +61,10 @@ class Juicescript {
 		Parse given PROGRAM-STRING and store syntax tree
 	*/
 	parse(program_string){
+		// CLEAR OLD PROGRAM TREE //
+		this.program_tree = null;
+		
+		
 		// DO SCANNING //
 		// get lexer
 		let lexer = new Juicescript_lexer(program_string, {
@@ -69,10 +73,29 @@ class Juicescript {
 		
 		// run lexical analysis
 		let token_list = lexer.scan();
-		/**/for(let one_token of token_list){
-			/**/one_token.type = Juicescript.token_type.name(one_token.type);
-			/**/console.log(one_token);
-		/**/}
+		
+		// stop here if unsuccessful
+		if(lexer.error_count > 0) return false;
+		
+		
+		// GET PROGRAM TREE FROM TOKENS //
+		// get parser
+		let parser = new Juicescript_parser(token_list, {
+			io: this.io
+		});
+		
+		// do parsing
+		let program_tree = parser.parse();
+		
+		// store program tree
+		this.program_tree = program_tree;
+		
+		// stop here if unsuccessful
+		if(parser.error_count > 0) return false;
+		
+		
+		// RETURN SUCCESS //
+		return true;
 	}
 	
 	/*
diff --git a/src/parser.js b/src/parser.js
new file mode 100644
index 0000000..ae03cad
--- /dev/null
+++ b/src/parser.js
@@ -0,0 +1,23 @@
+class Juicescript_parser {
+	/*
+		CONSTRUCTOR: Return new juicescript lexer for SOURCE with OPTIONS
+	*/
+	constructor(token_list, options){
+		// STORE ARGUMENTS //
+		// list of tokens
+		this.token_list = token_list;
+		
+		// io adapter
+		this.io = options.io;
+	}
+	
+	/*
+		MAIN: Do parsing
+	*/
+	parse(){
+		/**/for(let one_token of this.token_list){
+			/**/one_token.type = Juicescript.token_type.name(one_token.type);
+			/**/console.log(one_token);
+		/**/}
+	}
+}