You missed the return statement;
You should return it,so,react can render the page.

const lessonTableRowLink =  (LessonId:number|undefined) => {

        let link = '/edit/Lesson/'+LessonId;
        return <Redirect to= {link} /> // add return
    }

You should also check for NavLink or history#push

I’d recommend using react-router-dom‘s useHistory hook for this.

You’re code could look something like this:

import React, { FC } from 'react';
import { useHistory } from 'react-router-dom';

const MyComponent: FC = () => {
  const history = useHistory();

  const lessonTableRowLink = (LessonId:number|undefined) => {
    let link = '/edit/Lesson/'+LessonId;
    history.push(link);
  }

  return (
    ...
      <tr className="lessonTableRow" key={lesson.lessonId} onDoubleClick={() =>{ lessonTableRowLink(lesson.lessonId)}}>
    ...
  );
};

JSX doesn’t do anything on its own, and ESLint is warning you about this. Instead, you have to use React Router’s history API

history.replace(link)

I am trying to understand why this is returning an error Expected an assignment or function call and instead saw an expression @typescript-eslint/no-unused-expressions
as far as Im aware the value being passed into LessonId is correct.

Further upstream I am able to just <Redirect to="/edit/Lesson/"+LessonId; /> but im currently trying to redirect on doubleclick hence the function.

<tr className="lessonTableRow" key={lesson.lessonId} onDoubleClick={() =>{ lessonTableRowLink(lesson.lessonId)}}>

const lessonTableRowLink =  (LessonId:number|undefined) => {

        let link = '/edit/Lesson/'+LessonId;

        <Redirect to= {link} />
    }