How can I check if the input is empty in JavaScript?

This question already has an answer here:

How do you check for an empty string in JavaScript? 39 answers

---

function myFriday() {
  var input = document.getElementById("input1").value;

  var ever = function() {
    if (input.trim() == '') {
      return "Please input a number"
    } else if (!(isNaN(input))) {
      var result = 1;
      for (var i = 1; i <= input; i++) {
        result = result * i
      }
      return result;
    }

  }

  document.getElementById("input2").value = ever();
}

<p>Input:
  <input type="text" id="input1" />
</p>
<p>Input:
  <input type="text" id="input2" />
</p>
<button onclick="myFriday()">Calculate</button>
<p>RESULT: <span id="result" style="color:red"></span> 
</p>

---

你在找什么？

function myFriday() {
  var input = document.getElementById("input1").value;

  var ever = function () {
    if(input.match(/D/) == null){ // changes made here
      var result = 1;
      for(var i = 1; i <= input; i++ ) {
        result = result * i
      }
      return result;
    }
    else{ // one else is enough
      return "Please input a number"
    }
  }

  document.getElementById("input2").value = ever();
}

<p>Input: <input type="text" id = "input1" /></p>
<p>Input: <input type="text" id = "input2" /></p> 
<button onclick = "myFriday()">Calculate</button>
<p >RESULT: <span id = "result" style = "color:red"></span> </p>

---

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