Javascript check salesforce id validity

isValidSfId.js

function isValidSfId(str) {
    // https://stackoverflow.com/a/29299786/1333724
    if (typeof str !== 'string' || str.length !== 18) {
        return false;
    }

    let upperCaseToBit = (char) => char.match(/[A-Z]/) ? '1' : '0';
    let binaryToSymbol = (digit) => digit <= 25 ? String.fromCharCode(digit + 65) : String.fromCharCode(digit - 26 + 48);

    let parts = [
        str.slice(0,5).split("").reverse().map(upperCaseToBit).join(""),
        str.slice(5,10).split("").reverse().map(upperCaseToBit).join(""),
        str.slice(10,15).split("").reverse().map(upperCaseToBit).join(""),
    ];

    let check = parts.map(str => binaryToSymbol(parseInt(str, 2))).join("");

    return check === str.slice(-3);
}

Suggestions :

• add support for ids of 15 characters
• maybe add condition typeof str === "string"
• line 4 needs semicolon at end of line

Thanks @LoganTann ! I've implemented couple of your suggestions. Not sure about 15 characters Ids because the function check the checksum and this seems to be the part of 18 characters Ids only.

Organization IDs are mostly 15 characters, and is one of the most important IDs to validate? Is there any way to do that?