My tip/total cost calculator doesn't work right?

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Javascript

I originally had it figure in the tip (20% of bill) and tax (0.06) into the total of the meal and then confirm how much it would cost. Today I wanted to make it calculate the tax, and have the tip be optional (if at a fast food restaurant.) I used an if/ else if statement to do this, but it seems to display the tip no matter what. Any help would be appreciated, but please try and explain it simply as I am relatively new to coding; which probably means it is a simple mistake. The code is below. Thank you.

var meal = prompt("How much was your meal?")

var tax = 0.06
var tip = 0.20

var withTax = meal +++ meal * tax
confirm("Your total is" + " " + withTax)
  //problem starts here and ends at the else if
var yesNo = prompt("Would you like to include a tip?")
  //for some reason it always displays tip and won't run the else if
if (yesNo = "yes") {
  var total = withTax +++ withTax * tip

  confirm("Your total(plus tip) is" + " " + total)
}
//code below doesn't run?
else if (yesNo = "no") {

  confirm("Okay then, just" + " " + withTax)
}

What I have tried:

I have tried moving the if and else if around, and have looked it over several times. I am running the code in a website called https://jsfiddle.net/ if that helps.