SyntaxError: a declaration in the head of a for-of loop can't have an initializer

SyntaxError: for-of loop head declarations cannot have an initializer (Edge)
SyntaxError: a declaration in the head of a for-of loop can't have an initializer (Firefox)
SyntaxError: for-of loop variable declaration may not have an initializer. (Chrome)

let iterable = [10, 20, 30];
for (let value = 50 of iterable) {
  console.log(value);
}
// SyntaxError: a declaration in the head of a for-of loop can't
// have an initializer

let iterable = [10, 20, 30];
for (let value of iterable) {
  value += 50;
  console.log(value);
}
// 60
// 70
// 80