JavaScript Error Handling

Errors happen: a wrong input, a missing file, a lost network. If an error is not handled, the script stops. Error handling lets the program continue with a good message.

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try, catch and finally

try {
  console.log("Start");
  let result = undefinedVariable + 1;   // error
  console.log("This line is skipped");
} catch (error) {
  console.log("Error name: " + error.name);
  console.log("Message: " + error.message);
} finally {
  console.log("This always runs");
}

• try: the code that may fail.
• catch: runs only if an error happens. The error object has name and message.
• finally: always runs, good for cleanup.

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throw your own error

function setAge(age) {
  if (age < 0) {
    throw new Error("Age cannot be negative");
  }
  return age;
}

try {
  setAge(-5);
} catch (e) {
  console.log(e.message);
}

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Common error types

Error	When
ReferenceError	Using a variable that does not exist
TypeError	Using a value in a wrong way, like calling something that is not a function
SyntaxError	Code or JSON with wrong syntax
RangeError	A number outside the allowed range

try {
  null.name;
} catch (e) {
  console.log(e.name);
}

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Summary

• Put risky code in try and handle problems in catch.
• Use throw new Error("message") to raise your own errors.
• Next: JavaScript Events.

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