It is very common to check whether a number is within the bounds of an array or string using a comparison of form i < array.length, and later perform an indexing access array[i]. If this comparison is mistyped as i < array, a type coercion will be performed, which almost never has the intended effect.

Check if one of the operands is an array or a string, and make sure to compare against its length, not against the value itself.

The following example shows a mistyped loop condition i < array:

If the above is executed with array set to [3,5,7], the loop will not run at all. The error can be corrected by changing the loop condition to i < array.length:

Mozilla Developer Network: Array.length