JSFiddle - React, Tailwind, and code Playground

JavaScript

var MEM = [
    window, 'document', 'createElement', 'script',
    'text', 'alert(123)', 'body', 'appendChild'
];

var A, X, Y, Z

A = MEM[0]       //  window
X = MEM[1]       //  'document'
X = A[X]         //  X = window['document']

A = MEM[2]       //  'createElement'
Y = MEM[3]       //  'script'
A = X[A](Y)      //  A = document['createElement']('script')

Y = MEM[4]       //  'text'
Z = MEM[5]       //  'alert(123)'
A[Y] = Z         //  A['text'] = 'alert(123)'

Y = MEM[6]       //  'body'
X = X[Y]         //  X = document['body']

Y = MEM[7]       //  'appendChild'
X[Y](A)          //  body['appendChild'](A)