JSFiddle - React, Tailwind, and code Playground

JavaScript

var testArray = [ 'this one contains IN', 'this one does not' ];

function filterArray ( arr ) {
    var i = arr.length;
    while (i--) {
        if (arr[i].indexOf('IN') < 0) {
            arr.splice(i, 1);
        }
    }
}

filterArray( testArray );

document.body.innerText = JSON.stringify(testArray);