Why were Javascript `atob()` and `btoa()` named like that? - Stack Overflow

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In Javascript, window.atob() method decodes a base64 string and window.btoa() method encodes a string into base64.

Then why weren't they named like base64Decode() and base64Encode()? atob() and btoa() don't make sense because they're not semantic at all.

I want to know the reason.

In Javascript, window.atob() method decodes a base64 string and window.btoa() method encodes a string into base64.

Then why weren't they named like base64Decode() and base64Encode()? atob() and btoa() don't make sense because they're not semantic at all.

I want to know the reason.

• javascript

Share Improve this question Follow edited Oct 6, 2018 at 8:54 Константин Ван asked Nov 22, 2015 at 11:11 Константин ВанКонстантин Ван 14.7k88 gold badges7272 silver badges8686 bronze badges 5

• I too was convinced that atob and btoa were named backwards, being A the original string and B the encoded string, it was an unfortunate conicidence Base64 shared the initial with the encoded string B. Piling up on the confusion is the fact that I started using Linux only during the last decade, Linux provided the base64 program so I never had to know that btoa did the same. I hardly question naming choices, but after many years I just had to know. – Minkiele Commented Dec 12, 2019 at 11:32
• You could go function abes46neoced(a){return swab(swab(atob(a)))); but you would need to write your own swab function. – Stephen Duffy Commented Jul 10, 2020 at 1:39
• 2 It's because the 'b' in a atob stands for binary, not base64. ASCII is base64 encoded, and strings are binary. – Janac Meena Commented Jun 27, 2021 at 18:00
• 1 @janac ASCII is NOT base64, if anything, one could maybe argue base64 is a subset of ASCII. But that's not really true either, it just gets represented by alphanumeric symbols we all recognize. ASCII was originally designed with 7 bits aka base128 and the newer utf-8 format and now utf-16 are the most common. Base256 and base65536. But no one calls them that. Base64 was "created" and used to ensure older incompatible devices could talk, since some used 7bits and some used 6bits for network communication. Base64 is 6 bits so it worked on both. – Robert Cotterman Commented Sep 23, 2021 at 3:56
• As I guessed, they seem badly inverted ... as we think in 'B' as 'base64' not binary . BRILLIANT post indeed, thanks for sharing!! – Sergio Abreu Commented Nov 8, 2024 at 14:36

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5 Answers 5

Sorted by: Reset to default 274

I asked Brendan Eich (the creator of JavaScript) if he picked those names on Twitter and he responded:

Old Unix names, hard to find man pages rn but see https://www.unix.com/man-page/minix/1/btoa/ …. The names carried over from Unix into the Netscape codebase. I reflected them into JS in a big hurry in 1995 (after the ten days in May but soon).

In case the Minix link breaks, here's the man page content:

BTOA(1)                                           BTOA(1)

NAME
       btoa - binary to ascii conversion

SYNOPSIS
       btoa [-adhor] [infile] [outfile]

OPTIONS
       -a     Decode, rather than encode, the file

       -d     Extracts repair file from diagnosis file

       -h     Help menu is displayed giving the options

       -o     The obsolete algorithm is used for backward compatibility

       -r     Repair a damaged file

EXAMPLES
       btoa <a.out >a.btoa # Convert a.out to ASCII

       btoa -a <a.btoa >a.out
               # Reverse the above

DESCRIPTION
       Btoa  is  a  filter that converts a binary file to ascii for transmission over a telephone
       line.  If two file names are provided, the first in used for input and the second for out-
       put.   If  only one is provided, it is used as the input file.  The program is a function-
       ally similar alternative to uue/uud, but the encoding is completely different.  Since both
       of  these are widely used, both have been provided with MINIX.  The file is expanded about
       25 percent in the process.

SEE ALSO
       uue(1), uud(1).

The atob() and btoa() methods allow authors to transform content to and from the base64 encoding.

In these APIs, for mnemonic purposes, the "b" can be considered to stand for "binary", and the "a" for "ASCII". In practice, though, for primarily historical reasons, both the input and output of these functions are Unicode strings.

From : http://www.w3.org/TR/html/webappapis.html#atob

To sum up the already given answers:

• atob stands for ASCII to binary
  • e.g.: atob("ZXhhbXBsZSELCg==") == "example!^K"
• btoa stands for binary to ASCII
  • e.g.: btoa("\x01\x02\xfe\xff") == "AQL+/w=="

Why ASCII and binary:

• ASCII (the a) is the result of base64 encoding. A safe text composed only of a subset of ascii characters(*) that can be correctly represented and transported (e.g. email's body),
• binary (the b) is any stream of 0s and 1s (in javascript it must be represented with a string type).

(*) in base64 these are limited to: A-Z, a-z, 0-9, +, / and = (padding, only at the end) https://en.wikipedia.org/wiki/Base64

P.S. I must admit I myself was initially confused by the naming and thought the names were swapped. I thought that b stand for "base64 encoded string" and a for "any string" :D.

The names come from a unix function with similar functionality, but you can already read that in other answers here.

---

Here is my mnemonic to remember which one to use. This doesn't really answer the question itself, but might help people figure which one of the functions to use without keeping a tab open on this question on stack overflow all day long.

Beautiful to Awful btoa

Take something Beautiful (aka, beautiful content that would make sense to your application: json, xml, text, binary data) and transform it to something Awful, that cannot be understood as is (aka: encoded).

Awful to Beautiful atob

The exact opposite of btoa

Note

Some may say that binary is not beautiful, but hey, this is only a trick to help you.

I can't locate a source at the moment, but it is common knowledge that in this case, the b stands for 'binary', and the a for 'ASCII'.

Therefore, the functions are actually named:

ASCII to Binary for atob(), and Binary to ASCII for btoa().

Alos, note that this is browser implementation and was left for legacy / backward compatibility purposes. In Node.js, you would use:

Buffer.from("Hello World").toString('base64')
Buffer.from("SGVsbG8gV29ybGQ=", 'base64').toString('ascii')

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