SHA-256 with Javascript and Web Crypto

sha256.js

// Computes the SHA-256 digest of a string with Web Crypto
// Source: https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/digest

function sha256(str) {
  // Get the string as arraybuffer.
  var buffer = new TextEncoder("utf-8").encode(str)
  return crypto.subtle.digest("SHA-256", buffer).then(function(hash) {
    return hex(hash)
  })
}

function hex(buffer) {
  var digest = ''
  var view = new DataView(buffer)
  for(var i = 0; i < view.byteLength; i += 4) {
    // We use getUint32 to reduce the number of iterations (notice the `i += 4`)
    var value = view.getUint32(i)
    // toString(16) will transform the integer into the corresponding hex string
    // but will remove any initial "0"
    var stringValue = value.toString(16)
    // One Uint32 element is 4 bytes or 8 hex chars (it would also work with 4
    // chars for Uint16 and 2 chars for Uint8)
    var padding = '00000000'
    var paddedValue = (padding + stringValue).slice(-padding.length)
    digest += paddedValue
  }
  
  return digest
}

// Should output "c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2"
// We can check the result with:
// python -c 'from hashlib import sha256;print sha256("foobar").hexdigest()'
sha256("foobar").then(function(digest) {
  console.log(digest)
})