js addition script not working properly

Question

The following code should print the SUM of x and y. It works on the W3 editor, but not my Sublime? I don't understand why.

function add () {
    var x = prompt("Enter a number.");
    var y = prompt("Enter another number.");
    var a = x + y;
    alert(a);
    }

<script>
  add();
  </script>

The output I get is literally xy. If I enter x=1 and y=2, I get 12.
I also run into the same problem if I just hit the js code into the body without the function. I can't find my mistake. Could it be my editor?

prompt always returns a string. You've to convert the return values to numbers before the math. JS doesn't do automatic type coercion in this case, since + operator is used to concatenate strings too. — Teemu
– Teemu, Commented Jan 26, 2016 at 18:25
can you explain the difference, please? All 4 work. Under the hood there must differences (bytes used, etc?). — chignon
– chignon, Commented Jan 26, 2016 at 18:34
@chignon Here's a full reference to the JavaScript Number wrapper object, its properties and its methods: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — War10ck
– War10ck, Commented Jan 26, 2016 at 18:38

gurvinder372 · Accepted Answer · 2016-01-26 18:31:32Z

4

make it

function add () {
    var x =parseInt(prompt("Enter a number."),10);
    var y = parseInt(prompt("Enter another number."),10);
    if ( !isNaN(x) && !isNaN(y) )
    {
      var a = x + y;
      alert(a);
    }
    else
    {
      alert("One of the numbers is not valid");
    }
}

you need to parse the string received from the prompt to a integer first.

edited Jan 26, 2016 at 18:31

answered Jan 26, 2016 at 18:26

gurvinder372

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8 Comments

j08691 Over a year ago

Don't forget the radix.

War10ck Over a year ago

Also don't forget to check for NaN.

gurvinder372 Over a year ago

@j08691 and War10ck, thanks for pointing out, made the changes

chignon Over a year ago

checking for NaN I have difficulty with...example please?

gurvinder372 Over a year ago

@chignon NaN stands for Not a number. isNaN checks if the passed argument is a number or not.

|

War10ck · Accepted Answer · 2016-01-26 18:48:58Z

1

The values are strings so the + operator is treating it as string concatenation. Use parseInt() to generate integers, then do the addition:

function add () {
    // Best to include radix with parseInt
    var x = parseInt(prompt("Enter a number."), 10);
    var y = parseInt(prompt("Enter another number."), 10);
    // Quick and dirty check for NaN
    if(!(x != x) && !(y != y)) {
        var a = x + y;
        alert(a);
    } else {
        alert("Both inputs must be valid integers!");
    }
}

add();

Just for fun, here's an even shorter method:

function add () {
    var x = +prompt("Enter a number."),
        y = +prompt("Enter another number.");
    alert((!(x != x) && !(y != y)) ? x + y : "Both values must be integers!");
}

add();

edited Jan 26, 2016 at 18:48

answered Jan 26, 2016 at 18:28

War10ck

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Comments

anolsi · Accepted Answer · 2016-01-26 18:27:11Z

0

You can use use:

function add () {
    var x = Number(prompt("Enter a number."));
    var y = Number(prompt("Enter another number."));
    var a = x + y;
    alert(a);
}

answered Jan 26, 2016 at 18:27

anolsi

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Comments

xerotolerant · Accepted Answer · 2016-01-26 18:27:52Z

0

I think it is because prompt returns type string. Try using the

parseInt()

function. So

var x = parseInt(prompt("Enter a number."));

answered Jan 26, 2016 at 18:27

xerotolerant

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