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HTML 

              
                <h1>Prefill Machine</h1>

<p>
  <button id="prefill-correct">Prefill Correctly</button>
  <button id="prefill-incorrect">Prefill with Errors</button>
</p>

<form action="#0" id="form">
  
  <div>
    <label for="name">Name</label>
    <input type="text" name="name" id="name" placeholder="Digby Coyier" required>
  </div>
  
  <div>
    <label for="email">Email</label>
    <input type="email" name="email" id="email" placeholder="digby@digby.com" required>
  </div>
  
  <div>
    <label for="name">Username</label>
    <input type="text" name="username" id="username" placeholder="digby2007" required>
  </div>
  
  <div>
    <label for="name">Password</label>
    <input type="password" name="pw" id="pw" required>
  </div>
  
  <div>
    <label for="name">Repeat</label>
    <input type="password" name="pw-repeat" id="pw-repeat" required>
  </div>
  

  <fieldset>
    <legend>Radio Choice</legend>

    <div class="choice-group">
      <label class="radio-label" for="radio-choice-1">Choice 1</label>
      <input type="radio" name="radio-choice" id="radio-choice-1" value="choice-1" />

      <label class="radio-label" for="radio-choice-2">Choice 2</label>
      <input type="radio" name="radio-choice" id="radio-choice-2" value="choice-2" />
    </div>
  </fieldset>

  <div>
    <label for="select-choice">Select Choice</label>
    <div class="choice-group">
      <select name="select-choice" id="select-choice">
        <option value="Choice 1">Choice 1</option>
        <option value="Choice 2">Choice 2</option>
        <option value="Choice 3">Choice 3</option>
      </select>
    </div>
  </div>

  <div>
    <label for="message">Message</label>
    <textarea cols="40" rows="8" name="message" id="message"></textarea>
  </div>
  
  <div>
    <label for="cc">Credit Card #</label>
    <input type="text" name="cc" id="cc" placeholder="4242 4242 4242 4242" required>
  </div>

  <div>
    <label for="exp-1">Expiration <span class="screen-reader">Month</span></label>
    <input class="very-short" type="number" name="exp-1" id="exp-1" placeholder="08" min="1" max="12">
    <label for="exp-2" class="screen-reader">Expiration Year</label>
    <input type="number" name="exp-2" class="very-short" id="exp-2" placeholder="16" min="14">
  </div>

  <div>
    <label for="exp-1">CVV</label>
    <input class="short" type="text" name="cvv" id="cvv" placeholder="123">
  </div>
  
  <div>
    <label for="name">Address</label>
    <input type="text" class="long" name="address" id="address" placeholder="123 Super Street">
  </div>
  
  <div>
    <label for="city">&nbsp;<span class="screen-reader">City<span></label>
    <input type="text" name="city" id="city" class="medium"  placeholder="Milwaukee">
    <label for="state" class="screen-reader">State</label>
    <input type="text" name="state" class="very-short" id="state" placeholder="WI">
    <label for="zip" class="screen-reader">Zip</label>
    <input type="text" name="zip" class="short" id="zip" placeholder="55555" pattern="(\d{5}([\-]\d{4})?)" required>
  </div>
  
  <div>
    <label for="agree-terms">Agree?</label>
    <div class="choice-group">
      <input type="checkbox" name="agree-terms" id="agree-terms">
    </div>
  </div>

  <div>
    <input type="submit" value="Submit" id="submit-button">
  </div>
</form>
!

CSS 

              
                form {
  padding-top: 30px;
  min-width: 458px;
  > div,
  > fieldset {
    border: 0;
    padding: 0;
    margin: 0 0 8px 0;
    clear: both;
  }
  label,
  legend {
    float: left;
    width: 100px;
    padding: 7px 10px;
    &.radio-label {
      float: none;
      padding: 0;
    }
  }
  .choice-group {
    padding: 7px 10px 0 10px;
  }
  input[type=checkbox],
  input[type=radio] {
    margin-right: 10px;
  }
  input[type=text],
  input[type=email],
  input[type=password],
  input[type=number],
  textarea {
    width: 200px;
    border: 1px solid darken(tan, 20%);
    padding: 7px 10px;
    border-radius: 4px;
    outline: 0;
    &:focus {
      border-color: black;
    }
    &.short {
      width: 60px;
    }
    &.medium {
      width: 150px;
    }
    &.very-short {
      width: 40px;
    }
    &.long {
      width: 300px;
    }
  }
}

.screen-reader {
  position: absolute;
  top: -9999px;
  left: -9999px;
}

body {
  background: #E27C37;
  padding: 20px;
  font-size: small;
}
!

JS 

              
                var PrefillMachine = {
  
  init: function() {
    this.bindUIActions();
  },
  
  bindUIActions: function() {
    $("#prefill-correct").on("click", $.proxy(this.prefillCorrectly, this));
    
    $("#prefill-incorrect").on("click", $.proxy(this.prefillIncorrectly, this));
  },
  
  prefillCorrectly: function() {

    var id = this._makeId();

    // Prefixing only because would be easier to find in the database and purge.
    $("#name").val("name_" + id);
    $("#username").val("username_" + id);
    
    // The "+" syntax here is for GMail, which means I can use something unique but still receive the email for testing.
    $("#email").val("chriscoyier+" + id + "@gmail.com");
    
    // Match whatever password requirements you enforce here
    $("#pw, #pw-repeat").val("password_" + id);
    
    // Randomly select one or the other
    $("input[name='radio-choice']")
     .prop("checked", false)
     .eq(this._rand(0, 1))
     .prop("checked", true);
    
    // SELECT choice.
    $("#select-choice")
      .find("option")
      .prop("selected", false)
      .eq(this._rand(0, 2))
      .prop("selected", true);

    // The backslash-n's are new lines. Might as well test those come across OK.
    $("#message").val("\
There was an Old Man with a beard\n\
Who said, It is just as I feared!\n\
Two Owls and a Hen,\n\
Four Larks and a Wren,\n\
Have all built their nests in my beard!");
    
    // This is the Stripe card number for testing
    $("#cc").val("4242424242424242");
    // Expiration date can be anything as long as it's in the future
    $("#exp-1").val("08");
    $("#exp-2").val("15");
    // Can be anything with Stripe
    $("#cvv").val("343");
    
    $("#address").val(this._rand(1000, 9000) + " Super St.");
    $("#city").val("Milwaukee");
    $("#state").val("WI");
    $("#zip").val(this._rand(11111, 99999));
    
    $("#agree-terms").prop("checked", true);
  },

  prefillIncorrectly: function() {

    var id = this._makeId();

    // Empty should be invalid
    $("#name").val("");
    $("#username").val("");

    // Space should make invalid
    $("#email").val("jim @jim.com");

    // Force them not to match AND one is blank.
    $("#pw").val("123");
    $("#pw-repeat").val("");

    // Turn both off.
    $("input[name='radio-choice']")
     .prop("checked", false);

    // Can't really turn a select off but we can try.
    $("#select-choice")
      .find("option")
      .prop("selected", false);

    // Testing an empty-but-that's-OK thing.
    $("#message").val("");

    // Not enough numbers
    $("#cc").val("424242424242");
    // Values are wrong
    $("#exp-1").val("50");
    $("#exp-2").val("02");
    $("#cvv").val("abc");

    $("#address").val("");
    $("#city").val("");
    $("#state").val("");
    $("#zip").val("");

    // Left unchecked
    $("#agree-terms").prop("checked", false);

    // Triggering a submit event on the form seems to submit without validation
    // $("#submit-button").click();
  },

  _makeId: function()  {
    var text = "";
    var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

    for (var i=0; i < 5; i++) {
      text += possible.charAt(Math.floor(Math.random() * possible.length));
    }

    return text;
  },
  
  _rand: function(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
  }

}

PrefillMachine.init();
!
999px

Console