Why can I not implement a method here? The curly braces are underscored

I added an image for visibility but also here is the code:

function startAnimation() {
     setInterval( doAnimationStep, 40) // 40 ms = 25 frames per second
     this.stopAnimation= stopAnimation(){
       document.getElementById("buttonStop")= setInterval(doAnimationStep, 0)

     }
} ```

>Solution :

You can only have methods in classes and objects. However you can create functions within functions. Therefore you could change your code to the following and have similar behavior.

function startAnimation() {
  const setInterval = (doAnimationStep, 40) // 40 ms = 25 frames per second
  const stopAnimation = () => {
    document.getElementById("buttonStop")= setInterval(doAnimationStep, 0)
  }
}

On another note: I am not sure what you’re trying to accomplish with this line:

document.getElementById("buttonStop")= setInterval(doAnimationStep, 0)

setInterval() returns a non-zero number and getElementById() returns an Element and you’re assigning one to the other.