typeof在输入验证的if语句中失效？求排查JS代码问题（不使用正则）

Hey there! Let's break down why that typeof condition in your code isn't triggering like you expect.

The Core Issue: Input Values Are Always Strings #

Here's the key detail you might have missed: any value pulled from an input element's .value property is always a string—no matter what the user types. Even if someone enters 123, document.getElementById('demo').value will return the string "123", not a numeric type.

That means your check if(typeof xx === "number") will never be true. typeof "123" is always "string", so that throw statement never runs.

Fixing It Without RegExp #

Since you don't want to use regular expressions, here's how you can properly detect if the input is a numeric value:

1. Convert the input string to a number.
2. Check if that conversion resulted in a valid number (not NaN).
3. Optional: Verify the input was a pure numeric string (to avoid edge cases like "123abc").

Here's your updated code with these fixes:

function myDom(){ 
  var message = document.getElementById('show'); 
  var xx = document.getElementById('demo').value.trim(); // Trim whitespace to catch empty inputs with spaces
  try{ 
    if(xx === "") throw "can not be empty"; 
    // Convert input to a number
    const parsedNumber = Number(xx);
    // Check if the conversion gave us a valid number
    if(!isNaN(parsedNumber)) {
      // Optional: Ensure input was a pure numeric string (no extra characters)
      if(xx === parsedNumber.toString()) {
        throw "can not be number";
      }
    }
    // Uncomment if you want to add the <100 check
    // if(parsedNumber < 100 && !isNaN(parsedNumber)) throw "less than hundreds!" 
  }catch(err){ 
    message.innerHTML = "input " + err; 
  } 
}

Quick Explanation of the Fix #

• xx.trim(): Catches cases where the user enters only spaces (which would otherwise pass the empty check).
• Number(xx): Converts the string input to a number (returns NaN if the input isn't numeric).
• !isNaN(parsedNumber): Checks if the conversion worked (i.e., the input was a valid numeric value).
• xx === parsedNumber.toString(): Ensures the input didn't have extra non-numeric characters (like "123a"), which would fail this check even if Number(xx) returns NaN.

内容的提问来源于stack exchange，提问作者Wasiu