If-statement never reaches else branch - “if (x = y)”

问题

I have this weird problem:

Below is a JavaScript function

function calculateSum()
 {
   var e2 = document.getElementById('hotel_names');
   var selValue2 = e2.options[e2.selectedIndex].value;

   if (selValue2='1')
   {
   alert("helloworld");
   }
   else
   {
   alert("byeworld");
   }
//function closes
 }

It captures the option value of an HTML element selection and show the appropriate message...or it should be. The thing is it always shows 1 no matter the selection.

On the other hand the following works.

function calculateSum()
     {
       var e2 = document.getElementById('hotel_names');
       var selValue2 = e2.options[e2.selectedIndex].value;

       alert(selValue2);
     }

The second function shows the correct number each time you select an option from the selection. Any ideas why the first one does not work?

回答1:

Conditional statements use double equals sign called an equality operator -

  if (selValue2 == '1'){
    ...
  }

By using only one what you are essentially doing is assigning a value to the variable. This is called an assignment operator.

selValue2 = '1'

回答2:

You need an equality operator. You're using an assignment operator. Equality is

selValue2 == '1'

assignment is

selValue2 = '1'

回答3:

Just a small point to add to the other answers. I'd recommend using === instead of == for comparisons.

It checks both the value and the type of the parameters being compared and is often quicker.

来源：https://stackoverflow.com/questions/14369591/if-statement-never-reaches-else-branch-if-x-y

标签

javascript

if-statement