CodeQL documentation

Invalid prototype value

ID: js/invalid-prototype-value
Kind: problem
Severity: error
Precision: high
   - correctness
   - language-features
   - external/cwe/cwe-704
Query suites:
   - javascript-security-and-quality.qls

Click to see the query in the CodeQL repository

All JavaScript objects (including functions, classes and arrays) have a prototype, which is either null or another object. The prototype of an object can be set in two ways, both of which guard against attempts to assign an invalid prototype (such as a primitive value):

  1. Assigning a value to the object’s __proto__ property; if the value is not a valid prototype, the assignment is silently ignored.

  2. Using the standard library functions Object.create or Object.setPrototypeOf; invalid prototype values lead to a runtime error. In summary, any attempt to set the prototype of an object to a value that is not an object or null will be ineffective and may lead to a runtime error.


Fix the prototype assignment by providing a valid prototype value.


The following code attempts to create an object with prototype undefined, which will cause an error at runtime:

let dict = Object.create(undefined);

If the intention is to create an object without a prototype object, null should be used instead:

let dict = Object.create(null);