If checkbox checked, instantly alert

Question:

So, my goal is this: if the checkbox is checked by the user, do something (let's say just an alert). Here's my code, that is not working:

function validate() {
    if (document.getElementById('LetterNeed').checked) {
        alert("checked");
    } else {
        alert("You didn't check it! Let me check it for you.");
    }
}    

<input type="checkbox" name="LetterNeed" id="LetterNeed">Not important</span>

Thanks for your help!

Call the validate function on change of the checkbox state.

<input type="checkbox" name="LetterNeed" id="LetterNeed" onchange="return validate()">Not important</span>
//                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

OR

document.getElementById('LetterNeed').addEventListener('change', validate);

DEMO

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