Javascript sign concatenates instead of giving sum of variables

Question

Why when I use this   assuming i   1   divID    question-    i 1    I get question-11 and not question-2

User · Answer

var divID    question-     parseInt i  1     Use this   operator behave as concat that s why it showing 11

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Another alternative could be using  divID    quot question- quot     i - -1    Subtracting a negative is the same as adding  and a minus cannot be used for concatenation Edit  Forgot that brackets are still necessary since code is read from left to right

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divID    question-    parseInt i 1 10     check it here  it s a JSFiddle

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Since you are concatenating numbers on to a string  the whole thing is treated as a string  When you want to add numbers together  you either need to do it separately and assign it to a var and use that var  like this   i   i   1  divID    question-    i    Or you need to specify the number addition like this   divID    question-    Number i 1     EDIT  I should have added this long ago  but based on the comments  this works as well   divID    question-     i 1

User · Answer

Simple as easy     every input type if not defined in HTML is considered as string  Because of this the Plus     operator is concatenating   Use parseInt i  than the value of  i  will be casted to Integer    Than the     operator will work like addition   In your case do this  -   divID    question-    parseInt i  1

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Care must be taken that i is an integer type of variable  In javaScript we don t specify the datatype during declaration of variables  but our initialisation can guarantee that our variable is of a specific datatype   It is a good practice to initialize variables of declaration    In case of integers  var num   0  In case of strings  var str         Even if your i variable is integer    operator can perform concatenation instead of addition   In your problem s case  you have supposed that i   1  in order to get 2 in addition with 1 try using  i-1 2   Use of   -parenthesis will not be necessary   -  minus operator  cannot be misunderstood and you will not get unexpected result s

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Add brackets  divID    question-     i 1

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You may also use this   divID    question-     i 1 1      to be sure that i is converted to integer

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One place the parentheses suggestion fails is if say both numbers are HTML input variables  Say a and b are variables and one receives their values as follows  I am no HTML expert but my son ran into this and there was no parentheses solution i e     HTML inputs were intended numerical values for variables a and b  so say the inputs were 2 and 3  Following gave string concatenation outputs  a b displayed 23   a b displayed 23   a   b  displayed 23  From suggestions above we tried successfully   Number a  Number b  displayed 5  parseInt a    parseInt b  displayed 5    Thanks for the help just an FYI - was very confusing and I his Dad got yelled at  that is was Blogger com s fault  - no it s a feature of HTML input default combined with the  addition  operator  when they occur together  the default left-justified interpretation of all and any input variable is that of a string  and hence the addition operator acts naturally in its dual   parallel role now as a concatenation operator since as you folks explained above it is left-justification type of interpretation protocol in Java and Java script thereafter  Very interesting fact  You folks offered up the solution  I am adding the detail for others who run into this

User · Answer

Use this instead   var divID    question-     i 1    It s a fairly common problem and doesn t just happen in JavaScript  The idea is that   can represent both concatenation and addition   Since the   operator will be handled left-to-right the decisions in your code look like this     question-    i  since  question-  is a string  we ll do concatenation  resulting in  question-1   question-1    1  since  queston-1  is a string  we ll do concatenation  resulting in  question-11     With  question-     i 1  it s different    since the  i 1  is in parenthesis  its value must be calculated before the first   can be applied    i is numeric  1 is numeric  so we ll do addition  resulting in 2   question-    2  since  question-  is a string  we ll do concatenation  resulting in  question-2

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The reason you get that is the order of precendence of the operators  and the fact that   is used to both concatenate strings as well as perform numeric addition   In your case  the concatenation of  question-  and i is happening first giving the string  question 1   Then another string concatenation with  1  giving  question-11    You just simply need to give the interpreter a hint as to what order of prec endence you want   divID    question-     i 1

User · Answer

Joachim Sauer s answer will work in scenarios like this  But there are some instances where adding parentheses won   t help  For example  You are passing    sum of value of an input element and an integer    as an argument to a function  arg1      quot  elemId quot   val         value is treated as string arg2   1  someFuntion arg1   arg2         and so the values are merged here someFuntion  arg1   arg2        and here  You can make it work by using Number   arg1   Number    quot  elemId quot   val     arg2   1  someFuntion arg1   arg2    or arg1      quot  elemId quot   val    arg2   1  someFuntion Number arg1    arg2

User · Answer

using braces surrounding the numbers will treat as addition instead of concat   divID    question-     i 1

User · Answer

Use only   divID    question-    parseInt i    1    When  n  comes from html input field or is declared as string  you need to use explicit conversion   var n    1     type is string var frstCol   5  lstCol   frstCol   parseInt n     If  n  is integer  don t need conversion   n   1    type is int var frstCol   5  lstCol   frstCol   n

[javascript] Javascript (+) sign concatenates instead of giving sum of variables

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