js 의 조건문

var name = '';
if(name === 'zhangsan'){
	alert('    ');
}else{
	console.log('     ')
}

<label for="weather">Select the weather type today: </label>
<select id="weather">
  <option value="">--Make a choice--</option>
  <option value="sunny">Sunny</option>
  <option value="rainy">Rainy</option>
  <option value="snowing">Snowing</option>
  <option value="overcast">Overcast</option>
</select>

<p></p>

var select = document.querySelector('select');
var para = document.querySelector('p');

select.addEventListener('change', setWeather);

function setWeather() {
  var choice = select.value;

  if (choice === 'sunny') {
    para.textContent = 'It is nice and sunny outside today. Wear shorts! Go to the beach, or the park, and get an ice cream.';
  } else if (choice === 'rainy') {
    para.textContent = 'Rain is falling outside; take a rain coat and a brolly, and don\'t stay out for too long.';
  } else if (choice === 'snowing') {
    para.textContent = 'The snow is coming down — it is freezing! Best to stay in with a cup of hot chocolate, or go build a snowman.';
  } else if (choice === 'overcast') {
    para.textContent = 'It isn\'t raining, but the sky is grey and gloomy; it could turn any minute, so take a rain coat just in case.';
  } else {
    para.textContent = '';
  }
}

<label for="weather">Select the weather type today: </label>
<select id="weather">
  <option value="">--Make a choice--</option>
  <option value="sunny">Sunny</option>
  <option value="rainy">Rainy</option>
  <option value="snowing">Snowing</option>
  <option value="overcast">Overcast</option>
</select>

<p></p>

var select = document.querySelector('select');
var para = document.querySelector('p');

select.addEventListener('change', setWeather);


function setWeather() {
  var choice = select.value;

  switch (choice) {
    case 'sunny':
      para.textContent = 'It is nice and sunny outside today. Wear shorts! Go to the beach, or the park, and get an ice cream.';
      break;
    case 'rainy':
      para.textContent = 'Rain is falling outside; take a rain coat and a brolly, and don\'t stay out for too long.';
      break;
    case 'snowing':
      para.textContent = 'The snow is coming down — it is freezing! Best to stay in with a cup of hot chocolate, or go build a snowman.';
      break;
    case 'overcast':
      para.textContent = 'It isn\'t raining, but the sky is grey and gloomy; it could turn any minute, so take a rain coat just in case.';
      break;
    default:
      para.textContent = '';
  }
}

var greeting = (isBirthday)?'Happy birthday Mrs.Xiao': 'Today isnot my birthday';

--------------------------
<label for="theme">Select theme: </label>
<select id="theme">
  <option value="white">White</option>
  <option value="black">Black</option>
</select>

<h1>This is my website</h1>

var select = document.querySelector('select');
var html = document.querySelector('html');
document.body.style.padding = '10px';

function update(bgColor, textColor) {
  html.style.backgroundColor = bgColor;
  html.style.color = textColor;
}

select.onchange = function() {
  ( select.value === 'black' ) ? update('black','white') : update('white','black');
}

||   //  ：    ，           。
&&   //  ：    ，             。
!    //  :     ，             。