function load(textarea,path) {
    fs.readFile(path, (err, data) => {
        textarea.value=data;
    });
}

load(…,…);

const CodeMirror=require('codemirror');
const editor=CodeMirror.fromTextArea(textarea,{lineNumbers: true});

function load(textarea,path) {
    fs.readFile(path, (err, data) => {
        editor.setValue(data);
    });
}

load(…,…);

Uncaught TypeError: string.split is not a function