Salesforce ID checksum ... now in typescript!

SalesforceChecksum.ts

export class SalesforceChecksum
{
    private lookup = {};
    private checksumChars = Array.from("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345");

    constructor()
    {
        for (var i = 0; i < 32; i++)
        {
            let value = (i >>> 0).toString(2);
            let bit = ('00000'+value).substring(value.length);
            this.lookup[bit] = this.checksumChars[i];
        }
    }

    public validCheckSum(salesforceId: string): boolean
    {
        if (salesforceId.length != 18)
        return false;

        var chunks = [
            salesforceId.substring(0, 5),
            salesforceId.substring(5, 10),
            salesforceId.substring(10, 15)
        ];

        let checksum = salesforceId.substring(15, 18);
        let calculatedChecksum = "";
        chunks.forEach(chunk => {
            var reversed = chunk.split('').reverse();
            var resultString = "";
            for (var i = 0; i < reversed.length; i++)
            {
                if (!isNaN(reversed[i])){
                    resultString += "0";
                }
                else if (reversed[i] === reversed[i].toUpperCase()) {
                    resultString += "1";
                }
                else {
                    resultString += "0";
                }

            }
            calculatedChecksum += this.lookup[resultString];

        });
        return checksum === calculatedChecksum;
    }
}

SalesforceChecksum_Spec.ts

// jasmine tests

import {SalesforceChecksum} from "../SalesforceChecksum";

describe('when checking if a string is a valid salesforce ID', function () {
    let checksum = new SalesforceChecksum();

    it("this one is valid", ()=> {
       expect(checksum.validCheckSum("a0T5000000OV7X7EAL")).toBeTruthy();
    });

    it("and this one is valid", ()=> {
        expect(checksum.validCheckSum("a0Q5000000IdZj4EAF")).toBeTruthy();
    });

    it("even this one is valid", ()=> {
       expect(checksum.validCheckSum("a0Q5000000BlbfcEAB")).toBeTruthy();
    });

    it("but not this one is too short", ()=> {
        expect(checksum.validCheckSum("a0T5000000")).toBeFalsy();
    });

    it("and this one is too long", ()=> {
       expect(checksum.validCheckSum("a0T5000000123512dsadf")).toBeFalsy();
    });

    it("and this one isnt a valid checksum", ()=> {
        expect(checksum.validCheckSum("a0T5000000OV7X7EAX")).toBeFalsy();
    });

});