JSFiddle - React, Tailwind, and code Playground

JavaScript

var array = ['bla', ['ble', 'bli'], 'blo', ['blu']];
var flatten = function(arr) {
    var out = [];
    for(var i = 0; i < arr.length; i++) {
        out.push.apply(out, Array.isArray(arr[i]) ? flatten(arr[i]) : [ arr[i] ]);
    }
    
    return out;
};

console.log(array);
array = flatten(array); 
    
console.log(array);