Question

How can I do the following in JavaScript?

1. Concatenate “1”, “2”, “3” into “123”

2. Convert “123” into 123

3. Add 123 + 100 = 223

4. Covert 223 into “223”

Answer 1

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>