public ngOnInit() {
  const blah = [
    "division",
    [
      "blah",
      "blah1",
      [
        "division",
        "blah",
        [
          "division",
          "division",

        ]
      ],
      [
        "blah",
        "division",
        "blah"
      ]
    ]
  ];

  const flattened = this.reduce(blah);
  console.log(flattened);
}

public reduce(arr: any[]) {
  let out = [];

  arr.forEach((value, index) => {
    if (Array.isArray(value)) {
      out = out.concat(this.reduce(value));
    }

    if (value === 'division') {
      out = out.concat(value);
    }
  });

  return out;
}