javascript remove last word in textarea

Question

Can anyone help me to remove last word from the text area and it will replace with some another word.

Example:

I like my dog

should become

I like my cat

The last word is not always dog.

I'll update my code here

    function KeyCheck(e) {

var KeyID = (window.event) ? event.keyCode : e.keyCode;

switch(KeyID) {
    case 32:
        text2 = document.form1.box1.value;
        text2 = ReplaceLastWord(text2, "cat");
                alert(text2);
    }
}

function ReplaceLastWord(str, newStr) {
   return str.replace(/\w*$/, newStr);
}

I have put alert to check whether it's replaced or not

Answer 1

var str = 'I like my dog'; 
var newEndStr = 'cat'; 

function ReplaceLastWord(str, newStr) {
   return str.replace(/\w*$/, newStr);
}

console.log(ReplaceLastWord(str, newEndStr));

output:

I like my cat