I only want to run the function when isTrue is TRUE, but the function still runs when isTrue is false [duplicate]

I am trying to execute the function doSomething() ONLY if a global variable is TRUE. However, doSomething() still runs even when isTrue is FALSE. How come?

function doSomething() {
  console.log("DOING SOMETHING")
}

var isTrue = false

function doOrNotDo() {
  if (isTrue = true) {
    doSomething()
  }
  else {
    console.log("YO")
  }
}

doOrNotDo()

Even if I comment out the variable declaration isTrue, the code still runs doSomething ,for some weird reason I have yet to understand.

Please advice, thanks!