How to find and fix a type error of undefined property

Question

I'm following a tutorial on a crossword puzzle but after I trigger the selectLetter() function by pressing a key I'm getting this message when I run it in chrome:

"Uncaught TypeError: Cannot read property 'clueA' of undefined"*

and this message when I run it in firefox developer:

"TypeError: currentLetter.dataset is undefined".*

I tried using typeof in console.log to track the values of currentLetter and currentLetter.dataset but that's just confused me more.

 var allLetters; var currentLetter; var wordLetters; var acrossClue; var downClue; var typeDirection = "right"; window.onload = init; function init() { allLetters = document.querySelectorAll("table#crossword span"); currentLetter = allLetters[0]; var acrossID = currentLetter.dataset.clueA; var downID = currentLetter.dataset.clueD; acrossClue = document.getElementById(acrossID); downClue = document.getElementById(downID); formatPuzzle(currentLetter); for (var i = 0; i < allLetters.length; i++) { allLetters[i].style.cursor = "pointer"; allLetters[i].onmousedown = function(e) { formatPuzzle(e.target); } } document.onkeydown = selectLetter; console.log("----init function----\\n" + "Current Letter Type: " + typeof currentLetter + " \\ncurrentLetter.dataset Type: " + typeof currentLetter.dataset); } function formatPuzzle(puzzleLetter) { currentLetter = puzzleLetter; for (var i = 0; i < allLetters.length; i++) { allLetters[i].style.backgroundColor = ""; } acrossClue.style.color = ""; downClue.style.color = ""; if (currentLetter.dataset.clueA !== undefined) { acrossClue = document.getElementById(currentLetter.dataset.clueA); acrossClue.style.color = "blue"; wordLetters = document.querySelectorAll("[data-clue-a=" + currentLetter.dataset.clueA + "]"); for (i = 0; i < wordLetters.length; i++) { wordLetters[i].style.backgroundColor = "rgb(231,231,255)"; } } if (currentLetter.dataset.clueD !== undefined) { downClue = document.getElementById(currentLetter.dataset.clueD); downClue.style.color = "red"; wordLetters = document.querySelectorAll("[data-clue-d=" + currentLetter.dataset.clueD + "]"); for (i = 0; i < wordLetters.length; i++) { wordLetters[i].style.backgroundColor = "rgb(255,231,231)"; } } if (typeDirection === "right") { currentLetter.style.backgroundColor = "rgb(191,191,255)"; } else { currentLetter.style.backgroundColor = "rgb(255,191,191)"; } console.log("----formatPuzzle function----\\n" + "Current Letter Type: " + typeof currentLetter + " \\ncurrentLetter.dataset Type: " + typeof currentLetter.dataset); } function selectLetter(e) { var leftLetter = currentLetter.dataset.left; var upLetter = currentLetter.dataset.up; var rightLetter = currentLetter.dataset.right; var downLetter = currentLetter.dataset.down; var userKey = e.keyCode; console.log("----selectLetter function----\\n" + "Current Letter Type: " + typeof currentLetter + " \\ncurrentLetter.dataset Type: " + typeof currentLetter.dataset + "\\n.dataset.up:" + currentLetter.dataset.up + "\\nkeycode:" + userKey); if (userKey === 37) { formatPuzzle(leftLetter); } else if (userKey === 38) { formatPuzzle(upLetter); } else if (userKey === 39) { formatPuzzle(rightLetter); } else if ((userKey === 40) || (userKey === 13)) { formatPuzzle(downLetter); } else if ((userKey === 8) || (userKey === 46)) { currentLetter.textContent = ""; } else if (userKey === 32) { switchTypeDirection; } else if (65 <= userKey <= 90) { currentLetter.textContent = getChar(userKey); if (typeDirection === "right") { formatPuzzle(rightLetter); } else { formatPuzzle(downLetter); } } e.preventDefault(); } function switchTypeDirection() { } function getChar(keyNum) { return String.fromCharCode(keyNum); } 

 <table id="crossword"> <caption>Today's Crossword</caption> <tr> <td><span id="c1_1" data-letter="C" data-right="c1_2" data-left="c1_8" data-down="c2_1" data-up="c11_1" data-clue-a="a1" data-clue-d="d1"></span><sup>1</sup></td> <td><span id="c1_2" data-letter="A" data-right="c1_3" data-left="c1_1" data-down="c2_2" data-up="c11_2" data-clue-a="a1" data-clue-d="d2"></span><sup>2</sup></td> <td><span id="c1_3" data-letter="M" data-right="c1_4" data-left="c1_2" data-down="c2_3" data-up="c11_3" data-clue-a="a1" data-clue-d="d3"></span><sup>3</sup></td> <td><span id="c1_4" data-letter="O" data-right="c1_5" data-left="c1_3" data-down="c2_4" data-up="c10_4" data-clue-a="a1" data-clue-d="d4"></span><sup>4</sup></td> <td><span id="c1_5" data-letter="M" data-right="c1_6" data-left="c1_4" data-down="c2_5" data-up="c11_5" data-clue-a="a1" data-clue-d="d5"></span><sup>5</sup></td> <td><span id="c1_6" data-letter="I" data-right="c1_7" data-left="c1_5" data-down="c2_6" data-up="c11_6" data-clue-a="a1" data-clue-d="d6"></span><sup>6</sup></td> <td><span id="c1_7" data-letter="L" data-right="c1_8" data-left="c1_6" data-down="c2_7" data-up="c11_7" data-clue-a="a1" data-clue-d="d7"></span><sup>7</sup></td> <td><span id="c1_8" data-letter="E" data-right="c1_1" data-left="c1_7" data-down="c2_8" data-up="c11_8" data-clue-a="a1" data-clue-d="d8"></span><sup>8</sup></td> <td class="blank"></td> </tr> 

I expect the arrow keys to change the background color rgb(191,191,255) of the cell the arrow key directs to in the puzzle but the code stops running and a type error is printed to the console.

Answer 1

The error implies that currentLetter is assigned a value that is not a DOM element (because every DOM element has a dataset property).

The only places you assign to currentLetter is in init :

currentLetter = allLetters[0];

and formatPuzzle :

currentLetter = puzzleLetter;

allLetters[0] will be a DOM element.

puzzleLetter is the parameter of formatPuzzle , so lets see whether the function is called and passed something that is not a DOM element.

In selectLetter you are calling formatPuzzle multiple times with different arguments, but none of them is a DOM element. Eg leftLetter is the value of currentLetter.dataset.left .

Maybe you meant to do

var leftLetter = documnent.getElementById(currentLetter.dataset.left);
// etc

?

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Make sure you are only passing DOM elements to the function, and avoid global variables as much as possible.