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HTML 

              
                <main class="d-flex flex-column align-items-center">
    <h1>Password Generator</h1>
    
    <form class="form-inline">
        <input type="text" class="form-control" id="password" placeholder="Generate Password">
    </form>
    
    <form class="form-inline">
        <button type="button" class="btn btn-primary">Generate</button>
        <button type="button" class="btn btn-outline-primary">Copy</button>
    </form>
    
    <form class="form-inline">
        <div class="form-group">
            <label for="pwLength">Password Length</label>
            <select class="custom-select" id="pwLength">
                <option value="8">8</option>
                <option value="9">9</option>
                <option value="10">10</option>
                <option value="11">11</option>
                <option selected="12">12</option>
                <option value="13">13</option>
                <option value="14">14</option>
                <option value="15">15</option>
                <option value="16">16</option>
            </select>
            <label class="form-check-label">
            <input class="form-check-input" type="checkbox" id="caps">
                A-Z
            </label>
            <label class="form-check-label">
            <input class="form-check-input" type="checkbox" id="special">
                !-?
            </label>
            <label class="form-check-label">
            <input class="form-check-input" type="checkbox" id="numbers" checked="checked">
                1-9
            </label>
        </div>
    </form>
</main>
!

CSS 

              
                body {
    background-color: #E5E5E5;
    height: 100%;
}

main {
    padding-top: 50px;
    text-align: center;
    
    h1 {
        text-transform: uppercase;
        font-family: sans-serif;
        font-weight: 200;
    }
    
    button, select, label, input {
        margin: 0 0.5rem;
    }
    
    form {
        margin-top: 1rem;
    }
}
!

JS 

              
                document.addEventListener('DOMContentLoaded', function() {
    function copyToClipboard(text) {
        window.prompt('Copy to clipboard: Ctrl+C, Enter', text);
    }

    function generatePass(pwField) {
        var newPassword = '';
        var chars = 'abcdefghijklmnopqrstuvwxyz';
        var pwLength = document.getElementById('pwLength');
        var caps = document.getElementById('caps');
        var special = document.getElementById('special');
        var numbers = document.getElementById('numbers');

        if (caps.checked) {
            chars = chars.concat('ABCDEFGHIJKLMNOPQRSTUVWXYZ');
        }

        if (special.checked) {
            chars = chars.concat('!@#$%^&*');
        }

        if (numbers.checked) {
            chars = chars.concat('123456789');
        }

        for (var i = pwLength.value; i > 0; --i) {
            newPassword += chars[Math.round(Math.random() * (chars.length - 1))];
        }

        pwField.value = newPassword;
    }
    
    var gen = document.querySelector('.btn-primary');
    var copy = document.querySelector('.btn-outline-primary');
    var pwField = document.getElementById('password');
    
    // Why does this work but
    // gen.addEventListener('click', generatePass(pwField));
    // doesn't?
    gen.addEventListener('click', function() {
        generatePass(pwField);
    });
    
    copy.addEventListener('click', function() {
        copyToClipboard(pwField.value);
    });
})
!
999px

Console