Why is forEach function gives error " Cannot read property 'forEach' of undefined" here?

Question

Here is the source code

function MetadataParser(version, channel, keyField) {
  let _version = version;
  let _channel = channel;
  let _keyField = keyField;
  this.setVersion = function (version) {
    _version = version;
  };
  this.getVersion = function () {
    _version = version;
  };
  this.setChannel = function (channel) {
    _channel = channel;
  };
  this.getChannel = function () {
    return _channel;
  };
  this.setKeyField = function (keyField) {
    _keyField = keyField;
  };
  this.getVersion = function () {
    return _keyField;
  };
  this.getKeyFields = function (jsonObject) {
    let keyField = [];
    jsonObject.array.forEach((element) => {
      keyField.push(element.channel);
    });
    return keyField;
  };
} 
const metadataParser = new MetadataParser("A", "B", "C");
console.log(
  metadataParser.getKeyFields([
    { channel: "A" },
    { channel: "B" },
    { channel: "C" },
  ])
);

When I iterate the array using normal for loop the code works fine! The error occurs whenever i try to call the getKeyFields method.