JavaScript
let test = '<html><head></head><body>{"$id":"1","ContentEncoding":null,"ContentType":null,"Data":{"$id":"2","result":[{"$id":"3","hz":50.01,"vll":416.883,"cblk_avg_hz":49.968,"kw":248.987,"kwh_exp":0.0,"cblk_avg_mw":248.943,"kvar":-59.694,"kvab":-90.576,"meterid":1,"meter_name":"GT1","tstamp":"2019-04-24T16:42:19","CurrentTime":null,"Currenttstamp":null,"BlockNo":null,"blocktime":null},{"$id":"4","hz":50.01,"vll":417.50726,"cblk_avg_hz":49.96,"kw":284.7,"kwh_exp":0.0,"cblk_avg_mw":274.358,"kvar":80652.4,"kvab":26695.2,"meterid":5,"meter_name":"REACTOR","tstamp":"2019-04-24T16:42:19","CurrentTime":null,"Currenttstamp":null,"BlockNo":null,"blocktime":null},{"$id":"5","hz":50.01,"vll":423.682,"cblk_avg_hz":49.965,"kw":252.522,"kwh_exp":0.0,"cblk_avg_mw":252.537,"kvar":-59.89,"kvab":-89.998,"meterid":2,"meter_name":"GT2","tstamp":"2019-04-24T16:42:19","CurrentTime":null,"Currenttstamp":null,"BlockNo":null,"blocktime":null},{"$id":"6","hz":50.01,"vll":19.8810506666667,"cblk_avg_hz":49.962,"kw":275.362,"kwh_exp":0.0,"cblk_avg_mw":275.431,"kvar":27.596,"kvab":93.776,"meterid":3,"meter_name":"GEN1","tstamp":"2019-04-24T16:42:19","CurrentTime":null,"Currenttstamp":null,"BlockNo":null,"blocktime":null},{"$id":"7","hz":50.01,"vll":19.8406373333333,"cblk_avg_hz":49.958,"kw":271.482,"kwh_exp":0.0,"cblk_avg_mw":270.875,"kvar":19.877,"kvab":92.17,"meterid":4,"meter_name":"GEN2","tstamp":"2019-04-24T16:42:19","CurrentTime":null,"Currenttstamp":null,"BlockNo":null,"blocktime":null}]},"JsonRequestBehavior":1,"MaxJsonLength":null,"RecursionLimit":null}</body></html>'
let text = test.replace('<html><head></head><body>', '')
text = text.replace('</body></html>', '')
console.log(text)
console.log(JSON.parse(text))