Javascript Shuffle Array from Txt File

I am looking to pull 5 "random" lines from a text file while not repeating any. Each line from the text file has html code which would be inserted into a side menu. I have read up on the Fisher–Yates shuffle but not sure how to incorporate it in this manner with javascript. Presently I have the following throwing an error:

var request = new XMLHttpRequest();
request.onload = function() {
    var i = 0;
    // get the file contents
    var fileContent = this.responseText;
    // split into lines
    var fileContentLines = fileContent.split( '\n' );

    var target = document.getElementById( 'random-testimonial' );
    var targetHTML = target.innerHTML;

    while ( i < 5 ) {
        // get a random index (line number)
        var randomLineIndex = Math.floor( Math.random() * fileContentLines.length );
        // extract the value
        var randomLine = fileContentLines[ randomLineIndex ];

        // add the random line in a div if not duplicate            
        if ( ! targetHTML.contains(randomLine) ) {
            targetHTML += randomLine;
            i += 1;
        }
    }

    target.innerHTML = targetHTML;
};
request.open( 'GET', 'content.txt', true );
request.send();

and

<div id="random-content"><script src="content.js"></script></div>

Error:

content.js:19 Uncaught TypeError: targetHTML.contains is not a functionrequest.onload @ content.js:19