A Bookmarklet Calculator
If you think about it, it's really a bit too difficult to do a fullfledged calculator with buttons and a running value on just one long line of code. However, you can use a bookmarklet like Script 17.12 to do fairly complex calculations, using JavaScript's builtin Math functions, as described in Table 17.1.
Script 17.12. Surprisingly complex equations can be evaluated with this bookmarklet.
javascript:var evl,em,expr=prompt('Formula... (eg: 2*3 + 7/8)','');with(Math)try{evl=
parseFloat(eval(expr));if(isNaN(evl)) {throw Error('Not a number!');}void(prompt ('Result:'
,evl));}catch(em){alert(em);}

Table 17.1. JavaScript's Math FunctionalityFunction  Description 

abs  Absolute value  sin, cos, tan  Standard trigonometric functions; arguments in radians  acos, asin, atan  Inverse trigonometric functions; return values in radians  exp, log  Exponential and natural logarithm, base e  ceil  Returns least integer greater than or equal to argument  floor  Returns greatest integer less than or equal to argument  min  Returns lesser of two arguments  max  Returns greater of two arguments  pow  Exponential; first argument is base, second is exponent  round  Rounds argument to nearest integer  sqrt  Square root 
To use a JavaScript calculator:
1. 
javascript:var evl,em,expr=prompt ('Formula...(eg: 2*3 + 7/8)','');
This line sets up two variables and then prompts the user for an expression or formula, as shown in Figure 17.22, which is stored in expr.
 2. 
The next few lines need to be evaluated using JavaScript's builtin Math routines. The with(Math) line tells the interpreter that when any of these functions are seen, to evaluate them as Math commands.
The try{} warns JavaScript that what we're doing may well fail, and if so, don't panic. In fact, don't even put up an error message if there's a problem, as we'll be handling the errors ourselves.
   3. 
evl=parseFloat(eval(expr));
Evaluate the expression, and turn it into a floatingpoint number, which is then stored in evl.
 4. 
If the resulting value in evl is not a number ( NaN), do the following line.
 5. 
{throw Error('Not a number!');}
If we're here, for some reason what the user entered didn't work out to a number. When that happens we want to force an error message of "Not a number!" to display. Here's where the message is set; it will be displayed in step 7.
 6. 
void(prompt('Result:',evl));
Otherwise, the expression was valid, so display the result, as shown in Figure 17.23.
 7. 
Here's the end of that try{} block that started in step 2. To get here, one of two things happened: either we ran into the error in step 5, or some other error entirely occurred. Either way, we "catch" the error that was "thrown" and put it up on the screen in an alert.

Tip
