Guard

A guard is a type of conditional statement that is used in web programming languages like JavaScript and Ruby, to control the flow of a program or function. It is also referred to as a conditional expression, as it evaluates a condition and only executes the code within its block if the condition is true. Guards are a powerful tool in programming as they allow developers to write more concise and readable code.

Purpose of Guards

The primary purpose of a guard in web programming is to test a condition and control the flow of a program or function. It is used to check whether a certain condition is true, and if it is not, the guard returns an error or immediately exits the function, preventing further execution. Guards are especially useful when writing functions that require input validation or error handling.

Usage of Guards

Guards are used in programming languages like JavaScript and Ruby, to test a condition and control the flow of a program or function. They can be used in a variety of scenarios, such as input validation, error handling, and flow control. In the examples below, we will explore some common use cases for guards.

Input Validation

One common use case for guards is input validation. When writing functions that take input from users or external systems, it is important to validate that the input is in the correct format and meets certain criteria. Guards can be used to test the input and return an error if it does not meet the required criteria.

function calculateTax(price, taxRate) {
  if (typeof price !== 'number') {
    throw new TypeError('Price must be a number');
  }

  if (typeof taxRate !== 'number') {
    throw new TypeError('Tax rate must be a number');
  }

  return price * (1 + taxRate);
}

In this example, the calculateTax function takes two arguments: price and taxRate. The function uses guards to validate that both arguments are numbers. If either argument is not a number, the guard throws a TypeError and prevents further execution of the function. This ensures that the function only operates on valid input.

Error Handling

Guards are also useful for error handling. When writing functions that may encounter errors, it is important to catch and handle those errors. Guards can be used to catch certain types of errors and return a custom error message.

function divideNumbers(a, b) {
  if (b === 0) {
    throw new Error('Cannot divide by zero');
  }

  return a / b;
}

In this example, the divideNumbers function takes two arguments: a and b. The function uses a guard to check if b is equal to zero. If b is zero, the guard throws a custom error message, preventing division by zero. This ensures that the function always returns a valid result.

Flow Control

Guards can also be used for flow control. When writing code that has multiple conditions, guards can be used to simplify the logic and make the code more readable.

function checkStatus(status) {
  if (status === 'success') {
    return 'The operation was successful';
  }

  if (status === 'failure') {
    return 'The operation failed';
  }

  if (status === 'pending') {
    return 'The operation is pending';
  }

  throw new Error('Invalid status');
}

In this example, the checkStatus function takes one argument: status. The function uses guards to check the value of status and return a message based on the condition. If status is success, the guard returns a success message. If status is failure, the guard returns a failure message. If status is pending, the guard returns a pending message. If status is anything else, the guard throws an error. This ensures that the function always returns a valid message.