JavaScript string and number conversion

Question

How can I do the following in JavaScript    Concatenate  1    2    3  into  123  Convert  123  into 123 Add 123   100   223 Covert 223 into  223

User · Answer

Step  1  Concatenate  1    2    3  into  123     1     2     3    or     1    2    3   join       The join method concatenates the items of an array into a string  putting the specified delimiter between items  In this case  the  delimiter  is an empty string           Step  2  Convert  123  into 123   parseInt  123     Prior to ECMAScript 5  it was necessary to pass the radix for base 10  parseInt  123   10     Step  3  Add 123   100   223   123   100     Step  4  Covert 223 into  223     223  toString        Put It All Togther    parseInt  1     2     3     100  toString     or    parseInt   1    2    3   join        100  toString

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To convert a string to a number  subtract 0  To convert a number to a string  add     the empty string    5   1 will give you 6   5         1 will give you  51     5  - 0    1 will give you 6

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You want to become familiar with parseInt   and toString     And useful in your toolkit will be to look at a variable to find out what type it is   typeof    lt script type  text javascript  gt                 print out the value and the type of the variable passed in              function printWithType val            document write   lt pre gt             document write val           document write               document writeln typeof val           document write   lt  pre gt                var a    1   b    2   c    3   result          Step  1  Concatenate  1    2    3  into  123         - concatenation operator is just      as long          as all the items are strings  this works     result   a   b   c      printWithType result     123 string         - If they were not strings you could do     result   a toString     b toString     c toString        printWithType result      123 string         Step  2  Convert  123  into 123     result   parseInt result 10       printWithType result      123 number         Step  3  Add 123   100   223     result   result   100      printWithType result      223 number         Step  4  Convert 223 into  223      result   result toString           printWithType result      223 string         If you concatenate a number with a         blank string  you get a string         result   result           printWithType result     223 string  lt  script gt

User · Answer

Simplest is when you want to make a integer a string do  var a b  c  a   1  b   a toString       This will give you string   Now  from the variable b which is of type string we can get the integer  c   b  1    This will give you integer value of number  -    If you want to check above is a number  If you are not sure if b contains integer then you can use   if isNaN c 1         NOt a number   else   number

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You can do it like this      step 1  var one    1     2     3      string value  123      step 2 var two   parseInt one      integer value 123     step 3 var three   123   100     integer value 223     step 4 var four   three toString       string value  223

User · Answer

We can do this by using unary plus operator to convert them to numbers first and simply add  see below -   var a    4   var b    7   var sum    a    b

User · Answer

Below is a very irritating example of how JavaScript can get you into trouble   If you just try to use parseInt   to convert to number and then add another number to the result it will concatenate two strings     However  you can solve the problem by placing the sum expression in parentheses as shown in the example below   Result  Their age sum is  98  Their age sum is NOT  5048   x000D   x000D   lt  DOCTYPE html gt  x000D   lt html gt  x000D   lt body gt  x000D   x000D   lt p id  demo  gt  lt  p gt  x000D   x000D   lt script gt  x000D  function Person first  last  age  eye    x000D      this firstName   first  x000D      this lastName   last  x000D      this age   age  x000D      this eyeColor   eye  x000D    x000D   x000D  var myFather   new Person  John    Doe    50    blue    x000D  var myMother   new Person  Sally    Rally   48   green    x000D   x000D  document getElementById  demo   innerHTML    Their age sum is     x000D    parseInt myFather age  myMother age     Their age sum is NOT      x000D   parseInt myFather age  myMother age   x000D   lt  script gt  x000D   x000D   lt  body gt  x000D   lt  html gt  x000D   x000D   x000D

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parseInt is misfeatured like scanf    parseInt  12 monkeys   10  is a number with value  12    12 monkeys               is a number with value  NaN  Number  12 monkeys         is a number with value  NaN

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These questions come up all the time due to JavaScript s typing system  People think they are getting a number when they re getting the string of a number   Here are some things you might see that take advantage of the way JavaScript deals with strings and numbers  Personally  I wish JavaScript had used some symbol other than   for string concatenation   Step  1  Concatenate  1    2    3  into  123   result    1     2     3     Step  2  Convert  123  into 123  result     123     Step  3  Add 123   100   223  result   123   100    Step  4  Convert 223 into  223   result        223    If you know WHY these work  you re less likely to get into trouble with JavaScript expressions

User · Answer

r     1   2   3                step1   build string    gt   123  r    r                         step2   to number       gt  123 r   r 100                      step3    100            gt  223 r      r                       step4   to string       gt   223     in one line r          1   2   3   100

[javascript] JavaScript string and number conversion

Step (1) Concatenate "1", "2", "3" into "123"

Step (2) Convert "123" into 123

Step (3) Add 123 + 100 = 223

Step (4) Covert 223 into "223"

Put It All Togther

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