Javascript input run a function when specific text is typed in

Two issues:

1. = is for assignment not equality checking – you want to use ===.
2. You are storing the value of the input into readInput, so you don’t need to reference value from it:

function readCommand() {
  let readInput = document.querySelector('#commandInput').value
  //           ↓ no .value, it's already here -----------↑
  if (readInput === 'I want an ice cream sandwich') {
  //             ↑ === vs =
    giveIceCreamSandwich()
  } else {
    alert(`Not a command`);
  }
}