如何强迫 JS 做数学而不是把两个字符串放在一起

我需要 javascript 将5添加到一个整数变量，但是它将变量视为字符串，因此它写出变量，然后将5添加到“ string”的末尾。我怎么能强迫它做数学题呢？

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

输出: 55

如何强制它输出 10？

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最佳答案

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.

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the simplest:

dots = dots*1+5;

the dots will be converted to number.

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

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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.

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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;
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This also works for you: 

dots -= -5;
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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
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its really simple just

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

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

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

var dots = 5
function increase(){
dots = +dots + 5;
}
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After trying most of the answers here without success for my particular case, I came up with this:

dots = -(-dots - 5);

The + signs are what confuse js, and this eliminates them entirely. Simple to implement, if potentially confusing to understand.

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Number()

dots = document.getElementById("txt").value;
dots = Number(dots) + 5;



// from MDN
Number('123')     // 123
Number('123') === 123 /// true
Number('12.3')    // 12.3
Number('12.00')   // 12
Number('123e-1')  // 12.3
Number('')        // 0
Number(null)      // 0
Number('0x11')    // 17
Number('0b11')    // 3
Number('0o11')    // 9
Number('foo')     // NaN
Number('100a')    // NaN
Number('-Infinity') //-Infinity

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