Design a Text Editor LeetCode solution in JavaScript

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var TextEditor = function () {
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  this.forward = [];
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  this.backward = [];
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};
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/**
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 * @param {string} text
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 * @return {void}
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 */
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TextEditor.prototype.addText = function (text) {
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  for (let letter of text) {
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    this.forward.push(letter);
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  }
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};
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/**
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 * @param {number} k
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 * @return {number}
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 */
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TextEditor.prototype.deleteText = function (k) {
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  let deleted = 0;
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  while (this.forward.length && deleted < k) {
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    this.forward.pop();
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    deleted++;
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  }
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  return deleted;
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};
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/**
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 * @param {number} k
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 * @return {string}
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 */
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TextEditor.prototype.cursorLeft = function (k) {
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  let moved = 0;
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  while (this.forward.length && moved < k) {
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    this.backward.push(this.forward.pop());
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    moved++;
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  }
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  return toTheLeft(this.forward);
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};
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/**
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 * @param {number} k
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 * @return {string}
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 */
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TextEditor.prototype.cursorRight = function (k) {
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  let moved = 0;
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  while (moved < k && this.backward.length) {
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    this.forward.push(this.backward.pop());
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    moved++;
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  }
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  return toTheLeft(this.forward);
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};
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function toTheLeft(arr) {
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  let letters = [];
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  for (let i = Math.max(0, arr.length - 10); i < arr.length; i++) {
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    letters.push(arr[i]);
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  }
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  let res = letters.join("");
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  return res;
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}