it("Function insertAt(string, index) - converts the passed in string argument to an array and adds it at the given index", function (){
        let sb = new StringBuilder("Test");
        sb.insertAt("New", 2)
        assert.equal(sb._stringArray.join(""), "TeNewst");
    });
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it("Passing string to insertAt function should add to the existing array at the specified index", function() {
        let strBuilder = new StringBuilder("abc");
        strBuilder.insertAt("def", 1);
        let actual = strBuilder._stringArray;
        let expected = ["a", "d", "e", "f", "b", "c"];
        actual.forEach((x, i) => {
          assert.equal(x, expected[i]);
        });
      });