[Term Entry] JavaScript: Input Validation

content/javascript/concepts/input-validation/input-validation.md

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+---
+Title: 'Input Validation'
+Description: 'Input validation is the process of checking whether the data provided by a user or application is valid and meets the expected criteria before processing it further.'
+Keywords:
+  - input validation
+  - guard clause
+  - parameter validation
+  - error handling
+---
+
+# Input Validation
+
+Input validation is the process of checking whether data received from a user or external source is valid and meets the expected criteria before using it in an application. It's a critical security and reliability practice that helps prevent unexpected errors and protect against malicious input.
+
+## Why Input Validation Matters
+
+- **Error Prevention**: Catches invalid or unexpected data before it causes runtime errors
+- **Security**: Protects against injection attacks and malicious input
+- **Data Integrity**: Ensures the application works with clean, expected data
+- **Better User Experience**: Provides clear feedback when input doesn't meet requirements
+
+## Common Validation Checks
+
+- **Empty/Null Checks**: Verify that required parameters are not empty or undefined
+- **Type Checks**: Ensure data is of the expected type (string, number, array, etc.)
+- **Length Checks**: Verify strings meet minimum/maximum length requirements
+- **Format Checks**: Validate data matches expected patterns (email, phone, etc.)
+- **Range Checks**: Ensure numbers are within acceptable minimum and maximum values
+
+## Example: Guard Clause for Empty Parameters
+
+A simple but effective validation approach is using a guard clause to check for empty or undefined input at the beginning of a function:
+
+```javascript
+function parseContent(content) {
+  // Guard clause: validate input before processing
+  if (!content || content.trim() === '') {
+    throw new Error('Input content cannot be empty.');
+  }
+
+  // Safe to proceed with processing
+  return content.toUpperCase();
+}
+
+// Usage
+try {
+  parseContent(''); // Throws error
+} catch (error) {
+  console.error(error.message); // Output: Input content cannot be empty.
+}
+```
+
+## Best Practices
+
+1. **Validate Early**: Check input at the entry point of your function or module
+2. **Be Specific**: Provide clear, descriptive error messages that help users understand what went wrong
+3. **Fail Fast**: Stop processing immediately when validation fails
+4. **Document Requirements**: Clearly specify what constitutes valid input
+5. **Consider Edge Cases**: Handle null, undefined, empty strings, and unexpected types
+
+## See Also
+
+- [Try-Catch](/javascript/concepts/try-catch/)
+- [Error Handling](/javascript/concepts/errors/)
+- [Functions](/javascript/concepts/functions/)