OAuth 1.0a signature base string with form data

encodeBaseString.js

function percentEncode(string) {
  var char, charCode, i;
  var encodedString = '';

  for (i=0; i<string.length; i++) {
    char = string.charAt(i); 
    if ((char >= '0' && char <= '9') ||
      (char >= 'A' && char <= 'Z') ||
      (char >= 'a' && char <= 'z') ||
      (char == '-') || (char == '.') || 
      (char == '_') || (char == '~')) {
      encodedString += char;
    } else {
      charCode = string.charCodeAt(i);
      encodedString += '%' + charCode.toString(16).toUpperCase();
    }
  }
  return encodedString;
};

var dat = ''; // your (unparsed) form data here

'{method}&{url}&' + percentEncode(dat.split("&")
.map(a => a.split("=")
	.map(b => percentEncode(decodeURIComponent(b.replace(/\+/g,' ')))))
.filter(a => a[0] !== 'oauth_signature')
.sort((a, b) => {
    if (a[0] < b[0]) return -1;
    else if (a[0] == b[0]) return 0;
    return 1;
}).map(a => a[0] + '=' + a[1])
.join('&'))