countJSONleafNodes.js

var fs = require('fs');

var obj = JSON.parse(fs.readFileSync('fileURI', 'utf8'));

total = 0;
// obj is the root element; you can change <items> for whatever array you like
obj.items.forEach(element => {
    if(element.snippet.totalReplyCount>0){
        element.replies.comments.forEach(reply => {
            total++;
        })
    }
    total++;
});

console.log('\n' + total)