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JavaScript Errors

Errors Will Happen!

When executing JavaScript code, different errors can occur.

Errors can be coding errors made by the programmer, errors due to wrong input, and other unforeseeable things:

  • Syntax Errors
  • Reference Errors
  • Type Errors
  • Range Errors

How to Handle JavaScript Errors

The try statement allows you to define a block of code to be tested for errors while it is being executed.

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The JavaScript statements try and catch come in pairs:

try {
  Block of code to try
}
catch(err) {
  Block of code to handle errors
}

Syntax Errors

Syntax Errors occur when the code violates JavaScript's grammar rules.

ErrorExampleError
Unexpected tokenfname = "John);Invalid or unexpected token )
SyntaxErrorMath.round(4.6;Missing ) after argument list

Example

In this example we have made the mistake of not closing the string:

let fName = "John);
Try it Yourself »

The statement above will generate the error: Invalid or unexpected token

Execution of the program will stop!



Reference Errors

Reference Errors are triggered when referring a variable or aa function that does not exist.

ErrorExampleError
ReferenceErrorfname = foo; foo is not defined
Cannot access before initializationlet x = y;
let y = 5;
Cannot access y before initialization

Type Errors

Type Errors occur when a value is of the wrong type or an operation is invalid on that type.

ErrorExampleError
TypeErroranna();anna is not a function
Cannot read property of undefined/nulllet x = 5;
let y = x.length
Cannot read properties of undefined (reading 'lenght')

Range Errors

Type Errors occur when a numeric or iterable value is out of its valid range.

ErrorExampleError
RangeErrornew Array(-1);Invalid array length
ErrorExampleError
RangeErrornew Date("2025-02-33");Invalid date
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