JSFiddle - React, Tailwind, and code Playground

JavaScript

function merge(str, str2){
    var a = str.split(" ");
    str2.split(" ").forEach(function(i, index){
        var len = a.filter(function(item){return item == i;}).length;
        if(len == 0) a.push(i);
    });
    return a.join(" ");
}

console.log(merge("I am a sentence","I am a dancer"));