Letter case correction

Given a string, return the string with its first letter capitalized and lowercase for the rest.

Sample Code:

function letterCase(str) {
// Enter your logic here...
}

Examples:

Input: 'cODe'
Output: 'Code'
Explanation: Only the first letter is uppercase.

Input: 'Javascript'
Output: 'Javascript'
Explanation: Only the first letter is uppercase.