JavaScript: Create new string with first 3 characters are in lower case

Last update on June 30 2025 13:10:21 (UTC/GMT +8 hours)

JavaScript Basic: Exercise-37 with Solution

Modify String Based on Length (First 3 Lowercase/Uppercase)

Write a JavaScript program to produce a new string that has the first 3 characters in lower case from a given string. If the string length is less than 3 convert all the characters to upper case.

The program converts the first three characters of a given string to lowercase if the string length is 3 or more. If the string length is less than 3, it converts the entire string to uppercase.

Visual Presentation:

JavaScript: Create new string with first 3 characters are in lower case

Sample Solution:

JavaScript Code:

// Define a function named upper_lower with parameter str
function upper_lower(str) {
  // Check if the length of str is less than 3
  if (str.length < 3) {
    // If true, return the uppercase version of the entire string
    return str.toUpperCase();
  }
  
  // Create a variable named front_part and store the lowercase version of the first 3 characters of str
  front_part = (str.substring(0, 3)).toLowerCase();
  
  // Create a variable named back_part and store the substring of str from index 3 to the end
  back_part = str.substring(3, str.length);
  
  // Return the concatenation of front_part and back_part
  return front_part + back_part;
}

// Log the result of calling upper_lower with the argument "Python" to the console
console.log(upper_lower("Python"));

// Log the result of calling upper_lower with the argument "Py" to the console
console.log(upper_lower("Py"));

// Log the result of calling upper_lower with the argument "JAVAScript" to the console
console.log(upper_lower("JAVAScript"));

Output:

python
PY
javAScript

Live Demo:

Flowchart:

ES6 Version:

 // Define a function named upper_lower using arrow function syntax with parameter str
const upper_lower = (str) => {
  // Check if the length of str is less than 3
  if (str.length < 3) {
    // Return str converted to uppercase
    return str.toUpperCase();
  }

  // Create front_part as a lowercase substring of the first 3 characters of str
  const front_part = str.substring(0, 3).toLowerCase();
  // Create back_part as a substring of str from index 3 to the end
  const back_part = str.substring(3);

  // Return the concatenation of front_part and back_part
  return front_part + back_part;
};

// Log the result of calling upper_lower with the argument "Python" to the console
console.log(upper_lower("Python"));

// Log the result of calling upper_lower with the argument "Py" to the console
console.log(upper_lower("Py"));

// Log the result of calling upper_lower with the argument "JAVAScript" to the console
console.log(upper_lower("JAVAScript"));

For more Practice: Solve these Related Problems:

Write a JavaScript program that converts the first three characters of a string to lowercase if the string length is 3 or more, otherwise to uppercase.
Write a JavaScript program that changes the first three characters of a string to lowercase and the rest to uppercase if the string length exceeds 3.
Write a JavaScript program that modifies a string by altering the case of the first three characters and appending a suffix indicating the change.

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