Method #1. Here is the simple function to test if the string contains HTML data:
function isHTML(str) {
var a = document.createElement('div');
a.innerHTML = str;
for (var c = a.childNodes, i = c.length; i--; ) {
if (c[i].nodeType == 1) return true;
}
return false;
}
The idea is to allow browser DOM parser to decide if provided string looks like an HTML or not. As you can see it simply checks for ELEMENT_NODE (nodeType of 1).
I made a couple of tests and looks like it works:
isHTML('<a>this is a string</a>') // true
isHTML('this is a string') // false
isHTML('this is a <b>string</b>') // true
This solution will properly detect HTML string, however it has side effect that img/vide/etc. tags will start downloading resource once parsed in innerHTML.
Method #2. Another method uses DOMParser and doesn't have loading resources side effects:
function isHTML(str) {
var doc = new DOMParser().parseFromString(str, "text/html");
return Array.from(doc.body.childNodes).some(node => node.nodeType === 1);
}
Notes:
1. Array.from is ES2015 method, can be replaced with [].slice.call(doc.body.childNodes).
2. Arrow function in some call can be replaced with usual anonymous function.