Made the code more stable and resilient to wrong inputs, added example .html-file of how to use it

SAMPLE.js

@@ -2,10 +2,12 @@ var bibtexParse = require('bibtex-parse-js');
 
 var sample = bibtexParse.toJSON('@article{sample1,title={sample title}}');
 
-# Will conolse log:
+/*
+# Will console log:
 # 
 # [ { citationKey: 'SAMPLE1',
 #     entryType: 'ARTICLE',
 #     entryTags: { TITLE: 'sample title' } } ]
 #
+*/
 console.log(sample);

bibtexParse.js

@@ -173,12 +173,12 @@
                 return this.value_quotes();
             } else {
                 var k = this.key();
-                if (k.match("^[0-9]+$"))
-                    return k;
+                if (k.match("^[0-9]+\n*$"))
+                    return parseInt(k);
                 else if (this.months.indexOf(k.toLowerCase()) >= 0)
                     return k.toLowerCase();
                 else
-                    throw "Value expected: single_value" + this.input.substring(start) + ' for key: ' + k;
+                    throw "Value expected: single_value " + this.input.substring(start) + ' for key: ' + k;
 
             };
         };
@@ -190,7 +190,11 @@
                 this.match("#");
                 values.push(this.single_value());
             };
-            return values.join("");
+	    if(values.length == 1) {
+	        return values[0];
+	    } else {
+		return values.join("");
+	    }
         };
 
         this.key = function(optional) {
@@ -238,8 +242,16 @@
                     break;
                 }
                 ;
-                kv = this.key_equals_value();
-                this.currentEntry['entryTags'][kv[0]] = kv[1];
+		    try {
+			kv = this.key_equals_value();
+			this.currentEntry['entryTags'][kv[0]] = kv[1];
+		    } catch (e) {
+			    if(("" + e).includes("Value expected, equals sign missing: key_equals_value")) {
+				    console.warn("Warning: ignoring line");
+			    } else {
+				throw new Error(e);
+			    }
+		    }
             };
         };
 
@@ -302,7 +314,15 @@
                 } else {
                     this.entry(d);
                 }
-                this.match("}");
+		    try {
+			this.match("}");
+		    } catch (e) {
+			if(("" + e).includes("Token mismatch: match")) {
+				console.warn(e + ", ignoring line");
+			} else {
+				throw new Error(e);
+			}
+		    }
             };
 
             this.alernativeCitationKey();
@@ -311,6 +331,75 @@
 
     exports.toJSON = function(bibtex) {
         var b = new BibtexParser();
+	function cleanBibtex(bibtex_code) {
+		// searches for lines that should begin with @, but dont, and add @ for resiliency
+		var at_regex = /(\s+|^)(\w+\{)/ig;
+		bibtex_code = bibtex_code.replace(at_regex, (match) => `${match[1].replace(/[\n\r\R]*/, "")}\n@${match.replace(/[\n\r\R]*/, "")}`);
+
+		// remove spaces before @ at beginning of line
+		var space_regex = /^\s*@/g;
+		bibtex_code = bibtex_code.replace(space_regex, "@");
+
+		var search_missing_commas_regex_stage_one = /(.*=.*)/g;
+		bibtex_code = bibtex_code.replace(search_missing_commas_regex_stage_one, "$1,");
+
+		var search_missing_commas_regex_stage_two = /(,\s*)+\s*$/g;
+		bibtex_code = bibtex_code.replace(search_missing_commas_regex_stage_two, ",\n");
+
+		var find_unfinished_double_quotes = /(=\s*"[^"]*?),+/g;
+		bibtex_code = bibtex_code.replace(find_unfinished_double_quotes, `$1",`);
+
+		var find_unfinished_bracket_quotes = /(=\s*\{[^\}]*?),+/g;
+		bibtex_code = bibtex_code.replace(find_unfinished_bracket_quotes, `$1},`);
+
+		var find_missing_quotation = /(.*=)([^\{"'].*?[^\}"']),/g;
+		bibtex_code = bibtex_code.replace(find_missing_quotation, `$1{$2},`);
+
+		var find_missing_bracket_quote_start = /(.*=)([^\{].*?\}),/g;
+		bibtex_code = bibtex_code.replace(find_missing_bracket_quote_start, `$1{$2,`);
+
+		var find_missing_article_type = /^\s*(?<=@)([a-zA-Z0-9_]*)/g;
+		if(bibtex_code.match(find_missing_article_type)) {
+			bibtex_code = bibtex_code.replace(find_missing_article_type, `@article{$1`);
+			console.log("Found no article type, added @article");
+		}
+
+		var remove_double_commas = /,,+/g
+		bibtex_code = bibtex_code.replace(remove_double_commas, ",\n");
+
+		console.log(bibtex_code);
+
+		var lines = bibtex_code.split(/[\n\r]/);
+
+		function line_could_be_valid_syntax (line) {
+			if(line.match("^\s*@")) {
+				return true;
+			} else if(line.match("=")) {
+				return true;
+			} else if(line.match("}")) {
+				return true;
+			} else if(line.match(/^\s*$/)) {
+				return true;
+			} else {
+				return false;
+			}
+		}
+
+		var new_lines = [];
+
+		for (var i = 0; i < lines.length; i++) {
+			if(line_could_be_valid_syntax(lines[i])) {
+				new_lines.push(lines[i]);
+			} else {
+				console.warn("The line " + lines[i] + " does not seem to contain a valid bibtex line");
+			}
+		}
+
+		return new_lines.join("\n");
+	}
+
+	bibtex = cleanBibtex(bibtex);
+
         b.setInput(bibtex);
         b.bibtex();
         return b.entries;

example_site/index.html

@@ -0,0 +1,166 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>BibTeX Parser</title>
+</head>
+<body>
+    <script src="../bibtexParse.js"></script>
+
+    <h1>BibTeX to JSON Parser</h1>
+
+    <label for="bibtexInput">Enter BibTeX Code:</label>
+    <textarea id="bibtexInput" rows="5" cols="50">
+        @article{sample1,
+        title={sample title},
+        author="sample title"
+}
+
+@online{sample2,
+        title={sample title 2},
+        author="sample title 2",
+        year=1995,
+        url="https://google.de"
+}
+
+online{sample2,
+        title={sample title 2},
+        author="sample title 2"
+        year=1995,
+        url="https://google.de"
+}
+
+
+online{sample3,
+        title={sample title 3,
+        author="sample title 3"
+        year=19909,
+        url="https://google.de
+}
+
+online{sample4,
+        title=sample title 4,
+        author=sample title 4
+        year=199,
+        url="https://google.de
+}
+
+online{sample5,
+        title=sample title 5},
+        author=sample title 5}
+        year=199
+        url="https://google.de,,,
+
+        sfdfs
+
+        asdasdasd={12312
+}
+
+asdasdasdasd asd oaskfdösdfr234r 243r wef 
+
+@book{sample2,
+title={Buch ohne Autor},
+year={2019},
+}
+
+@article{sample#3,
+title={Unerwartete Zeichen im Schlüssel},
+author={Autor 3},
+year={2018},
+}
+
+@inproceedings{sample4,
+title={Titel mit falschen "Anführungszeichen"},
+author={Autor 4},
+year={2017},
+}
+
+@online{sample5,
+title={Ungültiges Feld},
+author={Autor 5},
+year={2021},
+url="https://example.com",
+publisher="Verlag 5",
+
+sample1,
+title={Fehlender Schlüssel},
+author={Autor 1},
+year={2022},
+url="https://example.com",
+}
+
+@book{sample6,
+title={Buch mit fehlendem Jahr},
+author={Autor 6},
+url="https://example.com",
+}
+
+@journal{sample7,
+title={Artikel mit falschem Eintragstyp},
+author={Autor 7},
+year={2015},
+}
+
+@article{sample8,
+title={Artikel mit Sonderzeichen wie !@# in Titel},
+author={Autor 8},
+year={2016},
+}
+
+@book{sample9,
+title={Buch 1},
+author={Autor 9},
+year={2020},
+}
+
+@book{sample9,
+title={Buch 2},
+author={Autor 10},
+year={2021},
+}
+
+@online{sample10,
+title={Online-Ressource mit unvollständiger URL},
+author={Autor 11},
+year={2019},
+url="example.com",
+}
+fsdfsdfd
+    </textarea>
+    <button onclick="parseBibtex()">Parse</button>
+
+    <h1>JSON</h1>
+    <pre id="outputDiv"></pre>
+    <h1>BibTeX</h1>
+    <pre id="bibtexDiv"></pre>
+    <div id="errorDiv" style="color: red;"></div>
+
+    <script>
+	function parseBibtex() {
+		try {
+			const bibtexInput = document.getElementById('bibtexInput').value;
+			const parsedJSON = bibtexParse.toJSON(bibtexInput);
+
+			if (parsedJSON) {
+				const prettyJSON = JSON.stringify(parsedJSON, null, 2);
+				document.getElementById('outputDiv').textContent = prettyJSON;
+				document.getElementById('bibtexDiv').textContent = bibtexParse.toBibtex(parsedJSON, 0);
+				document.getElementById('errorDiv').textContent = "";
+				return parsedJSON;
+			} else {
+			    document.getElementById('outputDiv').textContent = "Invalid BibTeX input.";
+			    document.getElementById('bibtexDiv').textContent = "";
+			    document.getElementById('errorDiv').textContent = "";
+			}
+		} catch (error) {
+			console.warn("Error parsing BibTeX:", error);
+			document.getElementById('outputDiv').textContent = "An error occurred while parsing the input.";
+			document.getElementById('bibtexDiv').textContent = "";
+			document.getElementById('errorDiv').textContent = "Error: " + error.message;
+		}
+
+		return {};
+	}
+    </script>
+</body>
+</html>
+