Throwing Your Own Errors

You can also throw your own errors by using a throw statement. The following code illustrates how to add throw statements to a function to throw an error, even if a script error does not occur. The function sqrRoot() accepts a single argument x. It then tests x to verify that it is a positive number and returns a string with the square root of x. If x is not a positive number, then the appropriate error is thrown, and the catch block returns the error:

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function sqrRoot(x) {
    try {
        if(x=="")    throw {message:"Can't Square Root Nothing"};
        if(isNaN(x)) throw {message:"Can't Square Root Strings"};
        if(x<0)      throw {message:"Sorry No Imagination"};
        return "sqrt("+x+") = " + Math.sqrt(x);
    } catch(err){
        return err.message;
    }
}
function writeIt(){
    console.log(sqrRoot("four"));
    console.log(sqrRoot(""));
    console.log(sqrRoot("4"));
    console.log(sqrRoot("-4"));
}
writeIt();

The following is the console output, showing the different errors that are thrown, based on input to the sqrRoot() function:

Can't Square Root Strings
Can't Square Root Nothing
sqrt(4) = 2
Sorry No Imagination