Allow origin function for AWS CloudFront

allow_origin.js

function allowOriginFunc(allowOrigins) {
  if (allowOrigins.length == 0) {
    return function(o) { return true; };
  }

  var origins = [], wildcards = [];

  allowOrigins.forEach(function(o) {
    var i = o.indexOf('*');
    if (i == -1) {
      origins.push(o)
    } else {
      wildcards.push([
        o.substring(0, i), // prefix
        o.substring(i+1), // suffix
      ]);
    }
  })

  return function(o) {
    if (!o) { return false; }
    for (var i = 0; i < origins.length; i++) {
      if (origins[i] == o) {
        return true;
      }
    }
    for (var i = 0; i < wildcards.length; i++) {
      if (o.hasPrefix(wildcards[i][0]) && o.hasSuffix(wildcards[i][1])) {
        return true;
      }
    }
    return false;
  };
}

String.prototype.hasPrefix = function(s) {
  return this.indexOf(s) == 0;
}

String.prototype.hasSuffix = function(s) {
  return this.indexOf(s, -1) == (this.length - s.length);
}

// configure allow origins
var allowOrigin = allowOriginFunc([
    "https://*.example2.io",
    "https://example.io",
    "http://localhost:*",
]);

function handler(event) {
    var request = event.request;
    var response = event.response;

    if (!response.headers['access-control-allow-origin']) {
        var origin = request.headers['origin'] || {};
        if (allowOrigin(origin.value)) {
            response.headers['access-control-allow-origin'] = origin;
        }
    }

    return response;
}