onClick работает нормально, вы должны проверить длину переменной guess следующим образом:
if (guess.length > 0) {
// ...
}
То, как вы делаете это в настоящее время, вы сравниваете (возможно, пустую) строку с числом (0), результаты могут быть не такими, как вы думаете, см. falsy и truey в JavaScript.
Примечание. Значение ввода всегда извлекается в виде строки, даже если оно содержит только цифры.
//create array for words
var listWords = ['cat', 'dog', 'mouse'];
//displays the word in underscores
var hiddenWord = [];
//choose word randomly
//Math.random returns integer between 0 (inclusive) and 1 (exclusive)
//multiply Math.random with the length of the listWords array
//Math.floor rounds down to nearest integer
//note that array indexes start counting from 0, hence why we need to make sure the listWords index never reaches the actual length of the array
var chosenWord = listWords[Math.floor(Math.random() * listWords.length)];
//number of tries the user gets before game over
var lives = 5;
//connected string of underscores that player sees in place of the chosenWord
var answerString;
//function creating underscores to be displayed in place of chosenWord
function hideWord() {
//for each letter in the chosenWord, replace it with an underscore in the new array
for (var i = 0; i < chosenWord.length; i++) {
hiddenWord[i] = '_';
}
//joins each underscore (element) of the hiddenWord array into a string with a space in between each
answerString = hiddenWord.join(' ');
//return the new string with spaces in between the underscores
return answerString;
}
function compareLetter() {
console.log('click')
//get the letter that the player typed in the text box
var guess = document.getElementById("guessedLetter").value;
console.log(guess);
//checking to see if player typed a letter
if (guess.length > 0) {
console.log('yes');
//compare the player input letter to the answer by moving through the chosenWord's array
for (var i = 0; i < chosenWord.length; i++) {
//if the player's letter matches one in the chosenWord, then display it
if (chosenWord[i] === guess) {
hiddenWord[i] = guess;
}
}
answerString = hiddenWord.join(' ');
document.getElementById('hiddenWord').innerHTML = answerString;
} else {
console.log('no');
}
}
//main function where actions are performed; where the other functions are called
function main() {
//creating the underscores to hide the chosenWord from the player
var underscores = document.getElementById("hiddenWord");
underscores.innerHTML = hideWord();
}
<!--run the main function from the javascript file when page is loaded-->
<body onLoad="javascript:main()">
<!--adding a title that will appear on the webpage-->
<h1>Hangman</h1>
<!--create a text box, restrict to only one letter being able to be typed, create placeholder text-->
<input id="guessedLetter" type="text" maxlength="1" minlength="1" placeholder="Guess a letter" />
<!--create a button to submit guessed letter and run the compareLetter function when clicked-->
<button type="button" onClick="compareLetter()">Guess!</button>
<!--underscores to hide the word that the player is guessing-->
<div id="hiddenWord"></div>
<!--add instructions for the player-->
<h2>Instructions</h2>
</body>