JavaScript：从图像中删除字节（base64字符串）

Question

I am working with Northwind service in SAP Web IDE. Images in this service is stored in base64 String format: FRwvAAIAAAAN.... .

I found out that I can't use these images in my app directly, using given base64 String value, because Northwind DB is old and made in MS Access and there are 78 redundant bytes which represent OLE header. So I would like to remove these 78 bytes from base64 String.

Can you please help me, using JavaScript language (I am new in this language). I hope for you experts. Here is what I have done:

I created function:

photo : function (value) { var str = ""; for (var p in value) { if (value.hasOwnProperty(p)) { str += value[p]; } } ..........

With this function I am taking base64 Sting as import parameter. I converted that import parameter from object to string.

So what should I do next? Create Array or something else? How can I remove 78 BYTES from String?

Answer 1

In base64 each character contains six bits of information, so four characters contains 24 bits of information, which is three bytes.

You are in luck. As 78 happens to be evenly divisble by three, the first 78 bytes corresponds exactly to the first 104 characters (78 bytes = 624 bits = 104 characters).

So, to remove the first 78 bytes of a base64 string, you remove the first 104 characters:

s = s.substr(104);

(If you hadn't been so lucky, you would have had to decode the entire string into bytes, remove the first 78 bytes, the encode the bytes into a string again.)