在JavaScript计算器中将NaN更改为Error

Question

I have a JavaScript calculator which has numerous functions defined as follows (using ln as an example):

function ln(form) {
  form.display.value = Math.log(form.display.value);
}

If the user inputs a negative number and clicks the button to activate this function, it returns NaN since ln is undefined for negative values. I would like for it to return Error however. I have tried this, but it causes the script to stop working:

var displayValue = form.display.value;
if (isNaN(displayValue)) displayValue = 'Error';

Here is the HTML markup of the display if required:

<INPUT NAME="display" ID="disp" VALUE="0" SIZE="28" MAXLENGTH="25"/>

How might I get this to return the desired output?

Answer 1

Try something like this:

function ln(form) {
    if(isNan(Math.log(form.display.value))){
       form.display.value = 'Error';
    } else {
      form.display.value = Math.log(form.display.value);
    } 
  }

Answer 2

The value returned from form.display.value is a String. This means that isNaN will always return true if the value cannot be parsed into a Number . You need to change this value to a Number first:

function ln(form) {
  var displayValue = parseFloat(form.display.value);

  if(isNaN(displayValue)){
    form.display.setAttribute('value', 'Error');
  } else {
    form.display.setAttribute('value', Math.log(displayValue));
  } 
}