What is the ‘Uncaught SyntaxError: Unexpected token’ Error?
Uncaught SyntaxError: Unexpected token is one of the most common syntax errors that occurs when JavaScript code does not follow the language’s syntax rules.
This error is thrown when the JavaScript engine encounters a character or keyword (token) that is grammatically incorrect and unexpected during the parsing process.
The error message usually appears in the form Unexpected token 'X', where 'X' points to the specific character or token that is causing the problem.
Main Causes
This error can be caused by a variety of syntax mistakes.
1. Mismatch of Parentheses, Braces, or Brackets
This occurs when an opening parenthesis is not properly closed, or when pairs do not match.
// The closing parenthesis should be ')', not '}'
if (condition) {
console.log("Hello, World!";
}
// Uncaught SyntaxError: Unexpected token '}'
2. Incorrect Operator Usage
Using = in an object literal or missing a comma can be a cause.
let myObject = {
key1: "value1",
key2 = "value2" // Should use ':', not '='
};
// Uncaught SyntaxError: Unexpected token '='
3. Incorrect Use of Reserved Words
This occurs when you try to use a reserved word like let, const, or class as a variable name.
let let = 5; // 'let' is a reserved word and cannot be used as a variable name
// Uncaught SyntaxError: Unexpected token 'let'
4. JSON Parsing Error
This error can occur when using JSON.parse() if the passed string is not a valid JSON format.
For example, if the keys in the JSON string are not wrapped in double quotes, or if there is a trailing comma after the last property.
// Error due to a trailing comma after the last property
let jsonString = '{"name": "John", "age": 30,}';
JSON.parse(jsonString);
// Uncaught SyntaxError: Unexpected token '}' in JSON at position ...
How to Fix It
1. Use a Code Editor and Linter
Most modern code editors (like VS Code, WebStorm, etc.) detect and highlight syntax errors in real-time. Additionally, using a linter tool like ESLint is very useful as it can find potential syntax errors before you even run the code.
2. Match Parentheses and Quotes
Check that all parentheses ((), {}, []) and quotes (', ", `) around the line where the error occurred are correctly paired and closed.
The bracket matching feature in code editors can help you find these easily.
3. Re-check Syntax Rules
Check what the Unexpected token in the error message is and double-check if the JavaScript syntax in that part is correct.
For example, check basic rules like defining object properties with a colon (:) and separating array elements and object properties with a comma (,).
4. Validate JSON
If the error is related to JSON.parse(), you need to check if the string you are trying to parse is in a valid JSON format.
You can use an online JSON validation tool (like JSONLint) or re-check the JSON rules (keys must always be in double quotes, no trailing comma after the last element, etc.).
Conclusion
Uncaught SyntaxError: Unexpected token is a clear signal that there is a syntax mistake in your code.
The first step to solving the problem is to carefully examine the location indicated by the error message and what the unexpected token is.
Actively using the syntax highlighting features of your code editor and a linter will be of great help in preventing and quickly fixing these types of errors.
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