The underscore symbol _ is a valid identifier in JavaScript, and in your example, it is being used as a function parameter.
A single underscore is a convention used by some javascript programmers to indicate to other programmers that they should “ignore this binding/parameter”. Since JavaScript doesn’t do parameter-count checking the parameter could have been omitted entirely.
This symbol is often used (by convention again) in conjunction with fat-arrow functions to make them even terser and readable, like this:
const fun = _ => console.log('Hello, World!')
fun()
In this case, the function needs no params to run, so the developer has used the underscore as a convention to indicate this. The same thing could be written like this:
const fun = () => console.log('Hello, World!')
fun()
The difference is that the second version is a function with no parameters, but the first version has a parameter called _ that is ignored. These are different though and the second version is safer, if slightly more verbose (1 extra character).
Also, consider a case like
arr.forEach(function (_, i) {..})
Where _ indicates the first parameter is not to be used.
The use of underscores like this can get very confusing when using the popular lodash or underscore libraries.