Non-Simple parameter lists

function a(x = 5) {
  "use strict";
}

is invalid JavaScript and will throw a SyntaxError because you cannot use the directive "use strict" in a function with Non-Simple Parameter list like the one above - default assignment x = 5

Non-Simple parameters include -

• Default assignemnt

function a(x = 1) {
  "use strict";
}

• Destructuring

function a({ x }) {
  "use strict";
}

• Rest params

function a(...args) {
  "use strict";
}