How to force JS to do math instead of putting two strings together

问题

I need javascript to add 5 to an integer variable, but instead it treats the variable as a string, so it write out the variable, then add 5 onto the end of the \"string\". How can I force it to do math instead?

var dots = document.getElementById(\"txt\").value; // 5
function increase(){
    dots = dots + 5;
}

Output: 55

How can I force it to output 10?

回答1:

You have the line

dots = document.getElementById("txt").value;

in your file, this will set dots to be a string because the contents of txt is not restricted to a number.

to convert it to an int change the line to:

dots = parseInt(document.getElementById("txt").value, 10);

Note: The 10 here specifies decimal (base-10). Without this some browsers may not interpret the string correctly. See MDN: parseInt.

回答2:

the simplest:

dots = dots*1+5;

the dots will be converted to number.

回答3:

DON'T FORGET - Use parseFloat(); if your dealing with decimals.

回答4:

I'm adding this answer because I don't see it here.

One way is to put a '+' character in front of the value

example:

var x = +'11.5' + +'3.5'

x === 15

I have found this to be the simplest way

In this case, the line:

dots = document.getElementById("txt").value;

could be changed to

dots = +(document.getElementById("txt").value);

to force it to a number

NOTE:

+'' === 0
+[] === 0
+[5] === 5
+['5'] === 5

回答5:

parseInt() should do the trick

var number = "25";
var sum = parseInt(number, 10) + 10;
var pin = number + 10;

Gives you

sum == 35
pin == "2510"

http://www.w3schools.com/jsref/jsref_parseint.asp

Note: The 10 in parseInt(number, 10) specifies decimal (base-10). Without this some browsers may not interpret the string correctly. See MDN: parseInt.

回答6:

This also works for you:

dots -= -5;

回答7:

its really simple just

var total = (1 * yourFirstVariablehere) + (1 * yourSecondVariablehere)

this forces javascript to multiply because there is no confusion for * sign in javascript.

回答8:

You can add + behind the variable and it will force it to be an integer

var dots = 5
    function increase(){
        dots = +dots + 5;
    }

回答9:

UPDATED since this was last downvoted....

I only saw the portion

var dots = 5
function increase(){
    dots = dots+5;
}

before, but it was later shown to me that the txt box feeds the variable dots. Because of this, you will need to be sure to "cleanse" the input, to be sure it only has integers, and not malicious code.

One easy way to do this is to parse the textbox with an onkeyup() event to ensure it has numeric characters:

<input size="40" id="txt" value="Write a character here!" onkeyup="GetChar (event);"/>

where the event would give an error and clear the last character if the value is not a number:

<script type="text/javascript">
    function GetChar (event){
        var keyCode = ('which' in event) ? event.which : event.keyCode;
        var yourChar = String.fromCharCode();
        if (yourChar != "0" &&
            yourChar != "1" &&
            yourChar != "2" &&
            yourChar != "3" && 
            yourChar != "4" &&
            yourChar != "5" &&
            yourChar != "6" && 
            yourChar != "7" &&
            yourChar != "8" && 
            yourChar != "9")
        {
            alert ('The character was not a number');
            var source = event.target || event.srcElement;
            source.value = source.value.substring(0,source.value-2);
        }
    }
</script>

Obviously you could do that with regex, too, but I took the lazy way out.

Since then you would know that only numbers could be in the box, you should be able to just use eval():

dots = eval(dots) + 5;

来源：https://stackoverflow.com/questions/4841373/how-to-force-js-to-do-math-instead-of-putting-two-strings-together

标签

javascript

string

math

addition