JavaScript string and number conversion

Question

How can I do the following in JavaScript?

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"

Answer 1

You can do it like this:

// step 1 
var one = "1" + "2" + "3"; // string value "123"

// step 2
var two = parseInt(one); // inegral value 123

// step 3
var three = 123 + 100; // integral value 223

// step 4
var four = three.toString(); // string value "223"

Answer 2

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

 "1" + "2" + "3"

Step (2) Convert "123" into 123

 parseInt("123", 10)

Step (3) Add 123 + 100 = 223

 123 + 100

Step (4) Covert 223 into "223"

 223.toString()

Put it all together:

 (parseInt("1" + "2" + "3", 10) + 100).toString()

Answer 3

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:

<script type="text/javascript">
    /**
     * print out the value and the type of the variable passed in
     */

    function printWithType(val) {
     document.write('<pre>');
     document.write(val);
     document.write(' ');
     document.writeln(typeof val);
     document.write('</pre>');
    }

    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
</script>

Answer 4

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.

Answer 5

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

Answer 6

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'