The Canada Under-20 international has signed until May 2019 after making 10 first-team appearances since his move from Vancouver Whitecaps in August.
The 18-year-old told Hearts TV: "I didn't expect it all to happen so quickly. I've just got on with it.
"I've started with the Under-20s and been progressing quite well recently, and moving forward into the first team has been really good. I'm really excited and really delighted to move forward so far.
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"I've enjoyed my year here so far and I'm just looking forward to carrying on with it and pushing forward."
Zanatta and his team-mates are looking to take their Tynecastle form on the road to Dundee on Saturday as they bid to follow up a hat-trick of home wins, which came after consecutive away losses to Dundee United and Hibernian.
Head coach Robbie Neilson said: "You want to win every game. Three home wins in a row was great but we need to back it up by going to Dens Park and getting a result."
Hearts will have United States international Perry Kitchen in their squad after the former DC United midfielder completed his move to Tynecastle this week - with the approval of national boss Jurgen Klinsmann.
And Neilson is delighted to have a player of his standing on board.
"Last season it would have been difficult being in the Championship and even this season if we were in the bottom six," Neilson said.
"But someone of Perry's calibre looks at the club and infrastructure that we are building here and the possibility of European football.
"He wanted to come to Europe to try to progress his international career. The US national manager wants players who have got European experience."
Neilson added: "He will be in the squad coming up to Dundee with us. It will probably take him a few weeks to get up to full fitness.
"The MLS finished in November and he has been in a training camp with the US national team for four weeks, but the last two or three weeks he hasn't done a lot because he has been travelling back and forth trying to get his visa sorted."
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