[OsamaAnsar]

Fixes # (no existing issue; the problem is described below)

What was wrong

Terms that are also property names of Object, such as constructor, were looked up on the prototype chain in lib/natural/tfidf/tfidf.js:

• TfIdf.tf is document[term] ? document[term] : 0. On a plain-object document that lacks the term, document['constructor'] is the Object function, so tf returns a function and tfidf() returns NaN.
• The constructor and addFileSync set this._idfCache = {}. With a plain-object cache, idf('constructor') finds Object in the cache and returns the function instead of a number. This hits instances created from deserialized data (new TfIdf(json)) and instances after addFileSync() (addDocument and removeDocument already use Object.create(null); issue Tfidf scoring doesnt work for the word 'constructor' #119 only patched the document-building side).

Real output on current master (3 documents: this document is about node and javascript, this document is about ruby and rails, the constructor is a special method):

call	before	after
tfidf('constructor', 0) (term absent)	NaN	0
tfidfs('constructor')	[NaN, NaN, 1.4054651081081644]	[0, 0, 1.4054651081081644]
tfidf('__proto__', 0)	NaN	0
deserialized: typeof idf('constructor')	function	number
deserialized: tfidf('constructor', 0) (absent)	NaN	0
deserialized: tfidf('constructor', 2) (present)	NaN	1.4054651081081644
after addFileSync: typeof idf('constructor')	function	number
after addFileSync: tfidf('constructor', 1) (present)	NaN	1

Ordinary terms (e.g. node) give identical results before and after. (tfidfs returns NaN; JSON.stringify of it prints null.)

Proposed Changes

• TfIdf.tf and documentHasTerm only consider own properties of the document (Object.prototype.hasOwnProperty.call).
• The constructor and addFileSync create the IDF cache with Object.create(null), as addDocument and removeDocument already do.
• Added Jasmine tests in spec/tfidf_spec.ts for: tf of an absent reserved name, tfidf of 0 (not NaN) for absent reserved names (toString, valueOf, hasOwnProperty, constructor, __proto__, ...), a present constructor term, listTerms with no NaN, a deserialized instance and the cache after addFileSync().

Verification

• With only the change in lib/natural/tfidf/tfidf.js reverted (confirmed with git diff --stat and grep), 4 of the new specs fail (the tf, tfidf of absent terms, deserialized and addFileSync ones); the other new specs are regression guards that pass either way. With the change restored, all 27 specs in tfidf_spec pass.
• npm test: 864 specs, 0 failures.
• eslint . --ext .ts (the lint script): no errors. npx standard lib/natural/tfidf/tfidf.js: no errors.
• npm run test_io was not run (it needs external services).

Checklist before merging

• Added Jasmine tests
• Updates TypeScipt index.d.ts files (not applicable: no API or type change, index.d.ts is untouched)