JavaScript Program: Catching and handling EvalError with the try-catch Block

JavaScript Error Handling: Exercise-11 with Solution

Handle EvalError

Write a JavaScript program that uses a try-catch block to catch and handle an 'EvalError' when evaluating an invalid expression.

Sample Solution:

JavaScript Code:

// Define a function named evaluate_Expression that takes a string expression (exp) as input
function evaluate_Expression(exp) {
  // Try block to evaluate the expression and handle potential errors
  try {
    // Evaluate the expression using the eval function
    const result = eval(exp);
    // Log the result to the console
    console.log('Result:', result);
  } catch (error) {
    // Catch block to handle errors
    // Check if the error is an instance of EvalError
    if (error instanceof EvalError) {
      // If the error is an EvalError, log the error message to the console
      console.log('EvalError:', error.message);
    } else {
      // If the error is not an EvalError, log the error message to the console
      console.log('Error:', error.message);
    }
  }
}

// Example:
// Call the evaluate_Expression function with a valid expression
evaluate_Expression('30 + 30'); // Valid expression
// Call the evaluate_Expression function with an invalid expression
evaluate_Expression('3 +'); // Invalid expression

Output:

"Result:"
60
"Error:"
"Unexpected end of input"

Note: Executed on JS Bin

Explanation:

In the above exercise -

The "evaluate_Expression()" function takes an expression as a parameter and evaluates it using the "eval()" function.

• Inside the try block, the code uses eval(exp) to evaluate the provided expression. The result is stored in the result variable.
• If the expression is valid and can be successfully evaluated, the result is logged to the console using console.log('Result:', result).
• If an error occurs during evaluation, the code jumps to the catch block.
• Inside the catch block, the error object is caught in the error parameter.
• The code checks if the error is an instance of EvalError using the instanceof operator.
• If it is, the error message is logged to the console as 'EvalError: <error message>'.
• If it's a different type of error, the error message is logged as 'Error: <error message>'.

Finally evaluate_Expression('30 + 30') returns a valid expression, '30 + 30', which results in 60 being logged to the console.

Next evaluate_Expression('3 +'), returns an invalid expression missing an operand. This causes an EvalError to be thrown. The error message, 'Unexpected end of input', is logged to the console.

Flowchart:

Live Demo:

See the Pen javascript-error-handling-exercise-11 by w3resource (@w3resource) on CodePen.

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