What is `_bokeh_timeout`?

What is _bokeh_timeout ? I searched for it but didn’t found any declaration .

$ grep -rnw './bokeh' -e '_bokeh_timeout'

./bokeh/core/_templates/autoload_nb_js.js:134:  if (typeof (root._bokeh_timeout) === "undefined" || force === true) {
./bokeh/core/_templates/autoload_nb_js.js:135:    root._bokeh_timeout = Date.now() + {{ timeout|default(0)|json }};
./bokeh/core/_templates/autoload_nb_js.js:164:    } else if (Date.now() < root._bokeh_timeout) {
./bokeh/core/_templates/autoload_nb_js.js:178:    } else if (Date.now() < root._bokeh_timeout) {

In JavaScript, you don’t have to declare a field in order to be able to use it. Line 134 checks if the field has already been set, and line 135 sets it if necessary.