Adding two numbers concatenates them instead of calculating the sum


I am adding two numbers, but I don't get a correct value.

For example, doing 1 + 2 returns 12 and not 3

What am I doing wrong in this code?

function myFunction() {
  var y = document.getElementById("txt1").value;
  var z = document.getElementById("txt2").value;
  var x = y + z;
  document.getElementById("demo").innerHTML = x;
}
<p>
  Click the button to calculate x.
  <button onclick="myFunction()">Try it</button>
</p>
<p>
  Enter first number:
  <input type="text" id="txt1" name="text1" value="1">
  Enter second number:
  <input type="text" id="txt2" name="text2" value="2">
</p>
<p id="demo"></p>

They are actually strings, not numbers. The easiest way to produce a number from a string is to prepend it with +:

var x = +y + +z;

I just use Number():

var i=2;  
var j=3;  
var k = Number(i) + Number(j); // 5  

You need to use javaScript's parseInt() method to turn the strings back into numbers. Right now they are strings so adding two strings concatenates them, which is why you're getting "12".


Use parseInt(...) but make sure you specify a radix value; otherwise you will run into several bugs (if the string begins with "0", the radix is octal/8 etc.).

var x = parseInt(stringValueX, 10);
var y = parseInt(stringValueY, 10);

alert(x + y);

Hope this helps!


Just add a simple type casting method as the input is taken in text. Use the following:

    var y = parseInt(document.getElementById("txt1").value);
    var z = parseInt(document.getElementById("txt2").value);
    var x = y + z;

Simple

var result = parseInt("1") + parseInt("2");
console.log(result ); // Outputs 3

The following may be useful in general terms.

  • First, HTML form fields are limited to text. That applies especially to text boxes, even if you have taken pains to ensure that the value looks like a number.

  • Second, JavaScript, for better or worse, has overloaded the + operator with two meanings: it adds numbers, and it concatenates strings. It has a preference for concatenation, so even an expression like 3+'4' will be treated as concatenation.

  • Third, JavaScript will attempt to change types dynamically if it can, and if it needs to. For example '2'*'3' will change both types to numbers, since you can’t multiply strings. If one of them is incompatible, you will get NaN, Not a Number.

Your problem occurs because the data coming from the form is regarded as a string, and the + will therefore concatenate rather than add.

When reading supposedly numeric data from a form, you should always push it through parseInt() or parseFloat(), depending on whether you want an integer or a decimal.

Note that neither function truly converts a string to a number. Instead, it will parse the string from left to right until it gets to an invalid numeric character or to the end and convert what has been accepted. In the case of parseFloat, that includes one decimal point, but not two.

Anything after the valid number is simply ignored. They fail if the string doesn’t even start off as a number. Then you will get NaN.

A good general purpose technique for numbers from forms is something like this:

var data=parseInt(form.elements['data'].value); //  or parseFloat

If you’re prepared to coalesce an invalid string to 0, you can use:

var data=parseInt(form.elements['data'].value) || 0;

This won't sum up the number; instead it will concatenate it:

var x = y + z;

You need to do:

var x = (y)+(z);

You must use parseInt in order to specify the operation on numbers. Example:

var x = parseInt(y) + parseInt(z); [final soulution, as everything us]

This code sums both the variables! Put it into your function

var y = parseInt(document.getElementById("txt1").value);
var z = parseInt(document.getElementById("txt2").value);
var x = (y +z);
document.getElementById("demo").innerHTML = x;`

You are missing the type conversion during the addition step...
var x = y + z; should be var x = parseInt(y) + parseInt(z);

 <!DOCTYPE html>

 <html>
 <body>
  <p>Click the button to calculate x.</p>
  <button onclick="myFunction()">Try it</button>
  <br/>
  <br/>Enter first number:
  <input type="text" id="txt1" name="text1">Enter second number:
  <input type="text" id="txt2" name="text2">
  <p id="demo"></p>
 <script>
    function myFunction() 
    {
      var y = document.getElementById("txt1").value;
      var z = document.getElementById("txt2").value;
      var x = parseInt(y) + parseInt(z);
      document.getElementById("demo").innerHTML = x;
    }
 </script>
 </body>
 </html>

  <input type="text" name="num1" id="num1" onkeyup="sum()">
  <input type="text" name="num2" id="num2" onkeyup="sum()">
  <input type="text" name="num2" id="result">

  <script>
     function sum()
     {

        var number1 = document.getElementById('num1').value;
        var number2 = document.getElementById('num2').value;

        if (number1 == '') {
           number1 = 0
           var num3 = parseInt(number1) + parseInt(number2);
           document.getElementById('result').value = num3;
        }
        else if(number2 == '')
        {
           number2 = 0;
           var num3 = parseInt(number1) + parseInt(number2);
           document.getElementById('result').value = num3;
        }
        else
        {
           var num3 = parseInt(number1) + parseInt(number2);
           document.getElementById('result').value = num3;
        }

     }
  </script>

It's very simple:

<html>

    <body>
        <p>Click the button to calculate x.</p>
        <button onclick="myFunction()">Try it</button>
        <br/>
        <br/>Enter first number:
        <input type="text" id="txt1" name="text1">Enter second number:
        <input type="text" id="txt2" name="text2">
        <p id="demo"></p>

        <script>
            function myFunction() {
                var y = document.getElementById("txt1").value;
                var z = document.getElementById("txt2").value;
                var x = +y + +z;
                document.getElementById("demo").innerHTML = x;
            }
        </script>
    </body>
</html>

Try this:

<!DOCTYPE html>
<html>

    <body>
        <p>Add Section</p>

        <label>First Number:</label>
        <input id="txt1"  type="text"/><br />
        <label>Second Number:</label>
        <input id="txt2"  type="text"/><br />

        <input type="button" name="Add" value="Add" onclick="addTwoNumber()"/>
        <p id="demo"></p>

        <script>
            function myFunction() {
                document.getElementById("demo").innerHTML = Date();
            }

            function addTwoNumber(){
                var a = document.getElementById("txt1").value;
                var b = document.getElementById("txt2").value;

                var x = Number(a) + Number(b);
                document.getElementById("demo").innerHTML = "Add Value: " + x;
            }
        </script>
    </body>
</html>

    <head>
        <script type="text/javascript">
            function addition()
            {
                var a = parseInt(form.input1.value);
                var b = parseInt(form.input2.value);
                var c = a+b
                document.write(c);
            }
        </script>
    </head>

    <body>
        <form name="form" method="GET">
        <input type="text" name="input1" value=20><br>
        <input type="text" name="input2" value=10><br>
        <input type="button" value="ADD" onclick="addition()">
        </form>
    </body>
</html>

If we have two input fields then get the values from input fields, and then add them using JavaScript.

$('input[name="yourname"]').keyup(function(event) {
    /* Act on the event */
    var value1 = $(this).val();
    var value2 = $('input[name="secondName"]').val();
    var roundofa = +value2+ +value1;

    $('input[name="total"]').val(addition);
});

You can do a precheck with regular expression wheather they are numbers as like

function myFunction() {
    var y = document.getElementById("txt1").value;
    var z = document.getElementById("txt2").value;
    if((x.search(/[^0-9]/g) != -1)&&(y.search(/[^0-9]/g) != -1))
      var x = Number(y)+ Number(z);
    else
      alert("invalid values....");
    document.getElementById("demo").innerHTML = x;
  }

Use parseFloat it will convert string to number including decimal values.

 function myFunction() {
      var y = document.getElementById("txt1").value;
      var z = document.getElementById("txt2").value;
      var x = parseFloat(y) + parseFloat(z);
      document.getElementById("demo").innerHTML = x;
    }


<p>
  Click the button to calculate x.
  <button onclick="myFunction()">Try it</button>
</p>
<p>
  Enter first number:
  <input type="text" id="txt1" name="text1" value="1">
  Enter second number:
  <input type="text" id="txt2" name="text2" value="2">
</p>
<p id="demo"></p>

You can also write : var z = x - -y ; And you get correct answer.

<body>

<input type="text" id="number1" name="">
<input type="text" id="number2" name="">
<button type="button" onclick="myFunction()">Submit</button>

<p id="demo"></p>

    <script>
    function myFunction() {
        var x, y ;

        x = document.getElementById('number1').value;
        y = document.getElementById('number2').value;

        var z = x - -y ;

        document.getElementById('demo').innerHTML = z;
    }
    </script>
</body>

Here goes your code by parsing the variables in the function.

<html>
  <body>
    <p>Click the button to calculate x.</p>
    <button onclick="myFunction()">Try it</button>
    <br/>
    <br/>Enter first number:
    <input type="text" id="txt1" name="text1">
    <br>Enter second number:
    <input type="text" id="txt2" name="text2">
    <p id="demo"></p>
    <script>
      function myFunction() {
        var y = parseInt(document.getElementById("txt1").value);
        var z = parseInt(document.getElementById("txt2").value);
        var x = y + z;
        document.getElementById("demo").innerHTML = x;
      }
    </script>
  </body>
</html>

Answer

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An alternative solution, just sharing :) :

var result=eval(num1)+eval(num2);