As we stroll our way down the winding path of our JavaScript Error Handling series, today we're stopping to smell the aroma of the Negative Repeat Count error. The Negative Repeat Count error, similar to the Infinite Repeat Count error, occurs when using the repeat() method of a String object in JavaScript, but in this case when the count parameter passed to that method is a negative value.
In this article we'll take a look at a few specific code examples that might produce a Negative Repeat Count error, and how to catch and handle this error when it appears. Here we go!
The Technical Rundown
- All JavaScript error objects are descendants of the Error object, or an inherited object therein.
- The RangeError object is inherited from the Error object.
- The Negative Repeat Count error is a specific type of RangeError object.
When Should You Use It?
Similar to the Infinite Repeat Count article, we're using a simple code example to illustrate when a Negative Repeat Counterror might appear. Below we have our count variable, which represents the number of repetitions of our String, and the name variable, which is our string to be repeated. Then we move on to calling the repeat() method and attempt to catch our expected RangeErrors using a simple printError function to format the error output:
var printError = function(error, explicit) {
console.log(`[${explicit ? 'EXPLICIT' : 'INEXPLICIT'}] ${error.name}: ${error.message}`);
}
try {
var count = -5;
var name = 'Bob';
name.repeat(count);
} catch (e) {
if (e instanceof RangeError) {
printError(e, true);
} else {
printError(e, false);
}
}
Since the count parameter we're passing here has a negative value of -5, as expected, we catch a Negative Repeat Counterror with our little example snippet.
// FIREFOX
[EXPLICIT] RangeError: repeat count must be non-negative// CHROME
[EXPLICIT] RangeError: Invalid count value
As with many other JavaScript errors, often the output messages will differ depending on the JavaScript engine (usually based on the browser in question), and here we see a clear example of this. Often the engine powering Chrome tends to be more explicit and verbose in its error reporting, but this is one exception (no pun intended) where Firefox takes that honor by explicitly stating that the repeat count value was a negative, whereas Chrome just indicates it is invalid, but fails to indicate why.
As it happens, that's all there is to the Negative Repeat Count error, since it only occurs explicitly when that count parameter value is a negative integer. The repeat() method itself is even smart enough to catch attempts to pass other object types, such as Strings, into the count parameter, and will automatically convert such a value to an integer before proceeding as normal. Here, we're trying to pass a String representation of a negative number, '-99', but JavaScript will have no part of these shenanigans:
var printError = function(error, explicit) {
console.log(`[${explicit ? 'EXPLICIT' : 'INEXPLICIT'}] ${error.name}: ${error.message}`);
}
try {
var count = '-99';
var name = 'Bob';
name.repeat(count);
} catch (e) {
if (e instanceof RangeError) {
printError(e, true);
} else {
printError(e, false);
}
}
Both browsers first convert this value to a number, then produce the same expected Negative Repeat Count error:
// FIREFOX
[EXPLICIT] RangeError: repeat count must be non-negative// CHROME
[EXPLICIT] RangeError: Invalid count value
To dive even deeper into understanding how your applications deal with JavaScript Errors, check out the revolutionary Airbrake JavaScript error tracking tool for real-time alerts and instantaneous insight into what went wrong with your JavaScript code.