Indian Air Force has no reasons to be afraid of the Chinese fighter J20. There are certain reasons for the same. Puneet Rastogi mentions the same on his answer on Quora.
1. Indian Air force is a professional competent unit, even if they think Chinese 5th generation Chengdu J-20 fighter jet is a possible threat to Indian security, they will make multiple counter plans and strategies to counter this threat. But any professional & reputed military will never panic because of just one weapon or weapon delivery system of adversary. India has already inducted a Brahmos Regiment in North east in the border near China. China does not have any counter measure for Bhramos ramjet supersonic cruise missiles, does this mean Chinese army/Air force should be in panic? China and India has always had a healthy rivalry but honestly I dont think there is any possibility of a hostile war or chances of any direct conflict in near future any ways. Our economic ties and dependency hardly allow for any military conflicts.
2. India possesses extremely potent and tested anti aircraft weaponry as well as air defense & early warning systems. India’s air defense network has two principal components- the Air Defense Ground Environment System(ADGES) and the Base Air Defense Zones(BADZ). The ADGES network provides for wide area radar coverage and permits the detection and interception of most aerial incursions into Indian airspace. The BADZ system, however, is far more concentrated with radars, interceptors, SAMs and AAA units working in conjunction to provide an intense and highly effective defensive barrier to attacks on vital targets.
Most Indian bases and vital targets are protected by full squadron of S-125 Pechora-1b SAMs and some critical spots are protected by additional deployments of OSA-AKM mobile SAMs. J-20 has never been pitched against Indian Air defense capabilities and Indian radar systems.
India is very close to acquire S400 triumf missile system from Russia which most analysts think is the best Surface to air missile system, its advance radar is extremely successful in detection of proven 5th Gen aircraft like russian T-50 and is expected to perform splendidly when pitched against F22 Raptor, F-35 Lightening and sure against Chinese J-20 as well. India has also sought proposals for advanced Aerial detection radar systems.
3. Most important point is that J-20 has not yet proved to be a very stealthy and successful fighter its self. There are considerable doubts over its stealth capabilities
Analysts have found out that the nose & canopy of J-20 use a similar stealth shaping design as the american F-22 raptor, yielding similar signature performance in a mature design at the front, however the aircraft’s side and axi-symmetric engine nozzles may expose the aircraft to radar. Some have even raised concerns about the use of canards on a low-observable design, stating that canards would guarantee radar detection and a compromise of stealth.
The fact that in spite of an indigenous 5th generation fighter, China is keen to import Russian made 4+ generation Su-35 aircraft tells us about the conviction of Chinese in J-20. While economics could be one reason behind this move, it does not bode well for future of J-20 Stealth fighter.
Considering the Indian current capabilities, future aspirations of Indian Military, the diplomatic relations with china and current track record of J-20 stealth fighter, I do not think Chengdu J-20 posses any significant threat to India.