IntToBase64.js

var digitsStr = 
//   0       8       16      24      32      40      48      56     63
//   v       v       v       v       v       v       v       v      v
    "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+-";
var digits = digitsStr.split(''),
    codes = digits.map( x => x.charCodeAt( 0 ) );

function base64id( int32 ){
    const a = digits[int32 & 0x3f];
    int32 >>>= 6;

    const b = digits[int32 & 0x3f];
    int32 >>>= 6;

    const c = digits[int32 & 0x3f];
    int32 >>>= 6;

    const d = digits[int32 & 0x3f];
    int32 >>>= 6;

    const e = digits[int32 & 0x3f];
    int32 >>>= 6;

    const f = digits[int32 & 0x3f];
    int32 >>>= 6;

    const g = digits[int32 & 0x3f];
    int32 >>>= 6;

    return digits[int32 & 0x3f] + g + f + e + d + c + b + a;    
}

function base64id2( int32 ){
    const a = codes[int32 & 0x3f];
    int32 >>>= 6;

    const b = codes[int32 & 0x3f];
    int32 >>>= 6;

    const c = codes[int32 & 0x3f];
    int32 >>>= 6;

    const d = codes[int32 & 0x3f];
    int32 >>>= 6;

    const e = codes[int32 & 0x3f];
    int32 >>>= 6;

    const f = codes[int32 & 0x3f];
    int32 >>>= 6;

    const g = codes[int32 & 0x3f];
    int32 >>>= 6;

    return String.fromCharCode(int32 & 0x3f, g, f, e, d, c, b, a );    
}