Base64 - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

Base64 is a group of similar binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding.

Base64 encoding schemes are commonly used when there is a need to encode binary data that needs to be stored and transferred over media that are designed to deal with ASCII. This is to ensure that the data remain intact without modification during transport. Base64 is commonly used in a number of applications including email via MIME, and storing complex data in XML.

One common application of base64 encoding on the web is to encode binary data  so it can be included in a data: URL.

In JavaScript there are two functions respectively for decoding and encoding base64 strings:

• btoa(): creates a base-64 encoded ASCII string from a "string" of binary data ("btoa" should be read as "binary to ASCII").
• atob(): decodes a base64 encoded string("atob" should be read as "ASCII to binary").

The algorithm used by atob() and btoa() is specified in RFC 4648, section 4.

Note that btoa() expects to be passed binary data, and will throw an exception if the given string contains any characters whose UTF-16 representation occupies more than one byte. For more details, see the documentation for btoa().

Encoded size increase

Each Base64 digit represents exactly 6 bits of data. So, three 8-bits bytes of the input string/binary file (3×8 bits = 24 bits) can be represented by four 6-bit Base64 digits (4×6 = 24 bits).

This means that the Base64 version of a string or file will be at most 133% the size of its source (a ~33% increase). The increase may be larger if the encoded data is small. For example, the string "a" with length === 1 gets encoded to "YQ==" with length === 4 — a 300% increase.