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Base64ToArrayBuffer.js
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47 lines (40 loc) · 1.88 KB
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// About base64: https://en.wikipedia.org/wiki/Base64
/**
* Converts a base64 string to an array of bytes
* @param {string} b64 A base64 string
* @returns {ArrayBuffer} An ArrayBuffer representing the bytes encoded by the base64 string
*/
function base64ToBuffer (b64) {
// The base64 encoding uses the following set of characters to encode any binary data as text
const base64Table = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
// Find the index of char '=' first occurrence
const paddingIdx = b64.indexOf('=')
// Remove padding chars from base64 string, if there are any
const b64NoPadding = paddingIdx !== -1 ? b64.slice(0, paddingIdx) : b64
// Calculate the length of the result buffer
const bufferLength = Math.floor((b64NoPadding.length * 6) / 8)
// Create the result buffer
const result = new ArrayBuffer(bufferLength)
// Create an instance of Uint8Array, to write to the `result` buffer
const byteView = new Uint8Array(result)
// Loop through all chars in the base64 string, in increments of 4 chars, and in increments of 3 bytes
for (let i = 0, j = 0; i < b64NoPadding.length; i += 4, j += 3) {
// Get the index of the next 4 base64 chars
const b64Char1 = base64Table.indexOf(b64NoPadding[i])
const b64Char2 = base64Table.indexOf(b64NoPadding[i + 1])
let b64Char3 = base64Table.indexOf(b64NoPadding[i + 2])
let b64Char4 = base64Table.indexOf(b64NoPadding[i + 3])
// If base64 chars 3 and 4 don't exit, then set them to 0
if (b64Char3 === -1) b64Char3 = 0
if (b64Char4 === -1) b64Char4 = 0
// Calculate the next 3 bytes
const byte1 = (b64Char1 << 2) + ((b64Char2 & 48) >> 4)
const byte2 = ((b64Char2 & 15) << 4) + ((b64Char3 & 60) >> 2)
const byte3 = ((b64Char3 & 3) << 6) + b64Char4
byteView[j] = byte1
byteView[j + 1] = byte2
byteView[j + 2] = byte3
}
return result
}
export { base64ToBuffer }