ReferenceError: invalid assignment left-hand side

if (Math.PI = 3 || Math.PI = 4) {
  console.log('no way!');
}
// ReferenceError: invalid assignment left-hand side

var str = 'Hello, '
+= 'is it me '
+= 'you\'re looking for?';
// ReferenceError: invalid assignment left-hand side

if (Math.PI == 3 || Math.PI == 4) {
  console.log("no way!");
}

var str = "Hello, " + "from the " + "other side!";