window对象相关

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    const x = window.matchMedia("(max-width: 400px)");
  
    function myFunction (x) {
      if (x.matches) {
        document.body.style.backgroundColor = 'red'
      } else {
        document.body.style.backgroundColor = 'blue'
      }
    }
    
    myFunction(x)
    x.addListener(myFunction);