Create algorithm for converting base64 to binary data

Author: nibble-4bits

Conversions/Base64ToArrayBuffer.js

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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 }

Conversions/test/Base64ToArrayBuffer.test.js

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+import { base64ToBuffer } from '../Base64ToArrayBuffer'
+import { TextDecoder } from 'util'
+
+describe('Base64ToArrayBuffer', () => {
+  it('should decode "SGVsbG8sIHdvcmxkIQ==" as "Hello, world!"', () => {
+    const testBase64String = 'SGVsbG8sIHdvcmxkIQ=='
+    const buffer = base64ToBuffer(testBase64String)
+    const decoder = new TextDecoder()
+    const helloWorldString = decoder.decode(buffer)
+    expect(helloWorldString).toBe('Hello, world!')
+  })
+
+  it('should decode base64 "Nxex6kQaWg==" as binary buffer [55,23,177,234,68,26,90]', () => {
+    const testBase64String = 'Nxex6kQaWg=='
+    const buffer = base64ToBuffer(testBase64String)
+    const array = [...new Uint8Array(buffer)]
+    expect(array).toEqual([55, 23, 177, 234, 68, 26, 90])
+  })
+
+  it('should decode base64 "AAECAwQFBgcICQ==" as binary buffer [0,1,2,3,4,5,6,7,8,9]', () => {
+    const testBase64String = 'AAECAwQFBgcICQ=='
+    const buffer = base64ToBuffer(testBase64String)
+    const array = [...new Uint8Array(buffer)]
+    expect(array).toEqual([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
+  })
+})