Convert mixed case string to lower case in JavaScript

In JavaScript, there are multiple ways to convert a mixed-case string to lowercase. The most common approach is using the built-in toLowerCase() method, but you can also implement a custom solution using character codes.

Using the Built-in toLowerCase() Method

The simplest and most efficient way is to use JavaScript's built-in toLowerCase() method:

const str = 'ABcD123';
const output = str.toLowerCase();
console.log(output);

abcd123

Custom Implementation Using Character Codes

For educational purposes, here's how to implement a custom convertToLower() function that converts uppercase letters (ASCII 65-90) to lowercase by adding 32 to their character codes:

const str = 'ABcD123';

String.prototype.convertToLower = function(){
    let res = '';
    for(let i = 0; i < this.length; i++){
        const el = this[i];
        const code = el.charCodeAt(0);
        if(code >= 65 && code <= 90){
            res += String.fromCharCode(code + 32);
        }else{
            res += el;
        };
    };
    return res;
};

console.log(str.convertToLower());

abcd123

How the Custom Method Works

The custom implementation works by:

• Iterating through each character in the string
• Getting the ASCII code using charCodeAt(0)
• Checking if the code is between 65-90 (uppercase A-Z)
• Adding 32 to convert uppercase to lowercase (A=65 becomes a=97)
• Keeping other characters unchanged

Comparison

Additional Examples

// Different test cases
const examples = ['Hello World!', 'MiXeD123CaSe', 'UPPERCASE', 'lowercase'];

examples.forEach(str => {
    console.log(`"${str}" ? "${str.toLowerCase()}"`);
});

"Hello World!" ? "hello world!"
"MiXeD123CaSe" ? "mixed123case"
"UPPERCASE" ? "uppercase"
"lowercase" ? "lowercase"

Conclusion

Use the built-in toLowerCase() method for production code as it handles Unicode characters properly and performs better. The custom implementation is useful for understanding how character conversion works at a lower level.