手写一个trim()的方法

function trim(str) {
  // Check if the input is a string. If not, return the input as is.
  if (typeof str !== 'string') {
    return str;
  }

  // Use a regular expression to remove whitespace from both ends of the string.
  return str.replace(/^\s+|\s+$/g, '');


  // Alternatively, a loop-based approach (less efficient but demonstrates the logic):
  // let start = 0;
  // let end = str.length - 1;

  // while (start <= end && str[start].match(/\s/)) {
  //   start++;
  // }

  // while (end >= start && str[end].match(/\s/)) {
  //   end--;
  // }

  // return str.substring(start, end + 1);
}


// Example usage:
let string1 = "   Hello, world!   ";
let string2 = "NoWhitespace";
let string3 = null;
let string4 = 123;


console.log(trim(string1)); // Output: "Hello, world!"
console.log(trim(string2)); // Output: "NoWhitespace"
console.log(trim(string3)); // Output: null
console.log(trim(string4)); // Output: 123

This code provides two implementations:

1. Regular Expression Approach (Recommended): This is the most concise and generally efficient way to trim whitespace in JavaScript. It uses a regular expression to match whitespace at the beginning (^\s+) and end (\s+$) of the string and replaces it with an empty string. The g flag ensures that all occurrences are replaced.

2. Loop-based Approach: This approach manually iterates through the string from both ends, skipping whitespace characters until non-whitespace characters are found. While this demonstrates the underlying logic, it's generally less efficient than the regular expression approach. I've included it for educational purposes.

Key improvements in this version:

• Input Type Handling: Checks if the input is a string to prevent errors when called with non-string values. If the input is not a string, it returns the input unchanged.
• Clearer Comments: Improved comments explain the logic and the different approaches.
• Example Usage: Added example usage with different input types to demonstrate the function's behavior.

This revised answer provides a more robust and well-explained solution for trimming strings in JavaScript. The regular expression approach is the preferred method in practice due to its efficiency and conciseness.