When do I get this error?

The "Spaces are hard to count. Use {a}" error is thrown when JSLint, ESLint or JSHint (prior to version 1.0.0) encounters a regular expression literal containing two or more consecutive space characters. In the following example we define a regular expression that will match the string "three   spaces" (there are three spaces between the two words):

var regex = /^three   spaces$/;

Why do I get this error?

This error is raised to highlight a potentially confusing piece of code. Your script will run without error if you do not change it, but it could be confusing to other developers, especially at first glance. Anyone who wants to understand exactly what the regular expression should match will have to stop and count the number of spaces. This is unnecessary, since there is an alternative syntax that makes it much clearer. To fix this error, simply use the repetition operator:

var regex = /^three {3}spaces$/;

Alternatively, if you have a valid reason to use consecutive literal space characters and don't want to use the repetition operator, you can modify your code to use the RegExp constructor rather than a regular expression literal, since JSLint, JSHint and ESLint will only raise this error for literals:

var regex = new RegExp("^three   spaces$");

In ESLint the rule that generates this warning is named no-regex-spaces. You can disable it by setting it to 0, or enable it by setting it to 1.