Variables that aren't defined, but accessed may throw reference errors at runtime.

Potential ReferenceErrors may result from misspellings of variable and parameter names, or accidental implicit globals (for example, forgetting the var keyword in a for loop initializer). Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment, or specified in the globals key in the configuration file. A common use case for these is if you intentionally use globals that are defined elsewhere (e.g. in a script sourced from HTML).

Bad Practice

const foo = someFunction();
const bar = a + 1; // 'a' is undeclared

Recommended

/*global someFunction, a*/
/*eslint no-undef: "error"*/

const foo = someFunction();
const bar = a + 1;