Validating strings before using them is crucial in JavaScript to prevent errors and unexpected behavior. Combining typeof, .trim(), and .length is a robust way to check if a variable is defined, is a string, and is not empty in JavaScript

 
function isValidString(str) {
    // Check if the variable is defined and is a string
    if (typeof str === 'string') {
        // Remove leading and trailing whitespace using .trim()
        const trimmedStr = str.trim();
        
        // Check if the trimmed string is not empty
        if (trimmedStr.length > 0) {
            return true; // It's a valid non-empty string
        }
    }
    return false; // It's either not a string or an empty string
}
 
// Example usage:
let myString = "  Hello, World!  ";
if (isValidString(myString)) {
    console.log("The string is valid and non-empty.");
} else {
    console.log("The string is either undefined, not a string, or empty.");
}
 

Explanation:

1. typeof str === 'string': This checks if the variable str is a string.
2. .trim(): This method removes leading and trailing whitespace from a string.
3. trimmedStr.length > 0: This checks if the length of the trimmed string is greater than 0, ensuring it's not empty.
4. isValidString() function encapsulates these checks, making it reusable for any string variable.

 
function isStringNotEmpty(str) {
  // Check if the variable is defined and is a string
  return typeof str === 'string' &&
 
  // Check if the trimmed string has any characters
  str.trim().length > 0;
}
 
// Example usage:
const string1 = "This is a string";
const string2 = "";
const string3 = null;
const string4 = 10;
 
console.log(isStringNotEmpty(string1)); // Output: true
console.log(isStringNotEmpty(string2)); // Output: false
console.log(isStringNotEmpty(string3)); // Output: false
console.log(isStringNotEmpty(string4)); // Output: false
 

Explanation:

1. typeof str === 'string': This part checks if the str variable is defined and is a string data type. The typeof operator returns the data type of a variable, and in this case, we're comparing it to the string literal "string".
2. .trim().length > 0: This part checks if the trimmed string has any characters. The .trim() method removes leading and trailing whitespace from the string, and .length returns the number of characters in the string. If the trimmed string has any characters, its length will be greater than 0.

This approach ensures that the str variable is a valid string and not empty before you use it in your code. You can use this function or similar logic wherever you need to validate strings in your JavaScript applications.