The code below takes input with a command and then appends it to the array. The result should show the history as if you kept typing and added it. I'm running into an issue where their extra comma appears right after the Hey on the 3rd iteration of the array and not 100% sure why that happens, I was able to remove some of the under commas by replacing but I'm not sure that right approach in this case.

class TextEditor {
    constructor(input) {
        this.input = input;
        this.output = [];
        this.history = [];
        this.forUndo = [];
        this.select = [];
    }
    textEditor() {
        let sortedInstructions = this.input.sort((a, b) => a[0] - b[0]);
        for (let instructor of sortedInstructions) {
            // Append
            if (instructor[0] === "APPEND") {
                if (this.select.length && this.output.length > 0) {
                    let last = this.output.pop();
                    let newLast =
                        last.slice(0, this.select[0]) +
                        instructor[1] +
                        last.slice(this.select[1]);
                    this.output.push(newLast);
                } else {
                    this.output.push(this.history + instructor[1]);
                    this.forUndo.push(instructor[1], "APPEND");
                    this.history.push(instructor[1]);
                }
            }
            return this. output;
        }
    }
}
let input = [["APPEND", "Hey"],
["APPEND", " you"],
["APPEND", ", don't"],
["APPEND", " "],
["APPEND", "let me down"]];

let t = new TextEditor(input);

console.log(t.textEditor());

let input = [["APPEND", "Hey"],
["APPEND", " you"],
["APPEND", ", don't"],
["APPEND", " "],
["APPEND", "let me down"]];

let t = new TextEditor(input);

console.log(t.textEditor());

output:

[
  'Hey',
  'Hey you',
  "Hey, you, don't",
  "Hey, you,, don't ",
  "Hey, you,, don't, let me down"
]

Expected:

[
  'Hey',
  'Hey you',
  "Hey you, don't",
  "Hey you, don't ",
  "Hey you, don't let me down"
]