Arrow function should not return assignment?

The Fix:

<div ref={(el) => { this.myCustomEl = el }} />

The Explanation:

Your current code is equivalent to:

<div ref={(el) => { return this.myCustomEl = el }} />

You are returning the result of this.myCustomEl = el. In your code, this is not really a problem — however, one of the most frustrating bugs in programming occurs when you accidentally use an assignment (=) instead of a comparator (== or ===), for instance:

// This function will always return **true**, surprisingly
function isEqual(a, b) {
  // The warning would be thrown here, because you probably meant to type "a===b". The below function will always return true;
  return a=b;
}

let k=false;
let j=true;

if(isEqual(k,j)){
  // You'll be very, very, very confused as to why this code is reached because you literally just set k to be false and j to be true, so they should be different, right? Right?
  thisWillExecuteUnexpectedly();
}

In the above case, the compiler warning makes sense because k=true evaluates to true (as opposed to k===true, which is probably what you meant to type) and causes unintended behavior. Thus, eshint notices when you return an assignment, assumes that you meant to return a comparison, and lets you know that you should be careful.

In your case, you can solve this by simply not returning the result, which is done by adding enclosing brackets {} and no return statement:

<div ref={(el) => { this.myCustomEl = el }} />

You can also adjust the eshint warning like so:
https://eslint.org/docs/rules/no-return-assign