function findOccurences(sentence, word){
    let regex = new RegExp(`\b${word}\b`, 'ig'); //case-insensitive and global flags
    console.log(sentence.match(regex).length);
}

findOccurences('The waterfall was so high, that the child couldn’t see its peak.', 'the');
findOccurences('How do you plan on achieving that? How? How can you even think of that?', 'how');
findOccurences('There was one. Therefore I bought it. I wouldn’t buy it otherwise.', 'there');