CodeQL documentation

Incorrect suffix check

ID: js/incorrect-suffix-check
Kind: problem
Security severity: 7.8
Severity: error
Precision: high
   - security
   - correctness
   - external/cwe/cwe-020
Query suites:
   - javascript-code-scanning.qls
   - javascript-security-extended.qls
   - javascript-security-and-quality.qls

Click to see the query in the CodeQL repository

The indexOf and lastIndexOf methods are sometimes used to check if a substring occurs at a certain position in a string. However, if the returned index is compared to an expression that might evaluate to -1, the check may pass in some cases where the substring was not found at all.

Specifically, this can easily happen when implementing endsWith using indexOf.


Use String.prototype.endsWith if it is available. Otherwise, explicitly handle the -1 case, either by checking the relative lengths of the strings, or by checking if the returned index is -1.


The following example uses lastIndexOf to determine if the string x ends with the string y:

function endsWith(x, y) {
  return x.lastIndexOf(y) === x.length - y.length;

However, if y is one character longer than x, the right-hand side x.length - y.length becomes -1, which then equals the return value of lastIndexOf. This will make the test pass, even though x does not end with y.

To avoid this, explicitly check for the -1 case:

function endsWith(x, y) {
  let index = x.lastIndexOf(y);
  return index !== -1 && index === x.length - y.length;


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