<form>
  <label for="email">メアド（必須）</label>
  <input 
    type="text"
    name="email"
    id="email"
    oninput="checkEmail()">
  <button 
    type="submit"
    id="button"
    disabled>送信</button>
</form>
function checkEmail() {
  const email = document.getElementById("email");
  const button = document.getElementById("button");
  if(email.value && email.value.length) {
    // 入力欄が空👉disabled解除
    button.disabled = false;
  } else {
    // 入力されている👉disabledを付与
    button.disabled = true;
  }
}

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