很简单的代码,用来做算法示范再好不过了,来自 一只猿

function Base64Encoding(input) 
{
	var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
	var output = "";
	var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
	var i = 0;

	input = utf8_encode(input);

	while (i < input.length) 
	{

		chr1 = input.charCodeAt(i++);
		chr2 = input.charCodeAt(i++);
		chr3 = input.charCodeAt(i++);

		enc1 = chr1 >> 2;
		enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
		enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
		enc4 = chr3 & 63;

		if (isNaN(chr2)) {
			enc3 = enc4 = 64;
		} else if (isNaN(chr3)) {
			enc4 = 64;
		}

		output = output +
		keyStr.charAt(enc1) + keyStr.charAt(enc2) +
		keyStr.charAt(enc3) + keyStr.charAt(enc4);

	}
 
	return output;
}