JavaScript Error Types

JavaScript has a few built-in error types. Knowing them helps you read error messages quickly.

ReferenceError

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try {
  console.log(missingVariable);
} catch (error) {
  console.log(error.name, "-", error.message);
  // → ReferenceError - missingVariable is not defined
}

Common causes: a typo in a name, using a variable outside its scope, using a let before its declaration.

TypeError

The most common one in day-to-day work.

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const user = null;

try {
  console.log(user.name);
} catch (error) {
  console.log(error.name);   // → TypeError
}

try {
  const notAFunction = 5;
  notAFunction();
} catch (error) {
  console.log(error.name);   // → TypeError
}

Typical messages:

• Cannot read properties of undefined (reading ‘x’): you used .x on undefined or null. Check where the value comes from, or use optional chaining.
• x is not a function: calling something that is not a function, often a typo or a missing return.
• Assignment to constant variable: you reassigned a const.

SyntaxError

The code itself is written wrongly, or JSON is invalid.

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try {
  JSON.parse("{'name': 'Maya'}");
} catch (error) {
  console.log(error.name);   // → SyntaxError
}

A syntax error in your script stops the whole script from running, so nothing in the file works. The console shows the line.

RangeError

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try {
  new Array(-1);
} catch (error) {
  console.log(error.name);   // → RangeError
}

try {
  (1.5).toFixed(200);
} catch (error) {
  console.log(error.name);   // → RangeError
}

Also thrown by infinite recursion: Maximum call stack size exceeded.

Checking the type

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try {
  null.x;
} catch (error) {
  if (error instanceof TypeError) {
    console.log("a type problem");  // → a type problem
  }
}

DOMException and others

Browser APIs throw their own errors, for example DOMException with names like NotFoundError, SecurityError, QuotaExceededError or AbortError. Check error.name for those.