SyntaxError: "use strict" not allowed in function with non-simple parameters

The JavaScript exception ""use strict" not allowed in function" occurs when a "use strict" directive is used at the top of a function with {{jsxref("Functions/Default_parameters", "default parameters", "", 1)}}, {{jsxref("Functions/rest_parameters", "rest parameters", "", 1)}}, or {{jsxref("Operators/Destructuring", "destructuring parameters", "", 1)}}.

Message#

SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list (V8-based)
SyntaxError: "use strict" not allowed in function with default parameter (Firefox)
SyntaxError: "use strict" not allowed in function with rest parameter (Firefox)
SyntaxError: "use strict" not allowed in function with destructuring parameter (Firefox)
SyntaxError: 'use strict' directive not allowed inside a function with a non-simple parameter list. (Safari)

Error type#

{{jsxref("SyntaxError")}}.

What went wrong?#

A "use strict" directive is written at the top of a function that has one of the following parameters:

• {{jsxref("Functions/Default_parameters", "Default parameters", "", 1)}}
• {{jsxref("Functions/rest_parameters", "Rest parameters", "", 1)}}
• {{jsxref("Operators/Destructuring", "Destructuring parameters", "", 1)}}

A "use strict" directive is not allowed at the top of such functions per the ECMAScript specification.

Examples#

Function statement#

In this case, the function sum has default parameters a=1 and b=2:

```js-nolint example-bad function sum(a = 1, b = 2) { // SyntaxError: "use strict" not allowed in function with default parameter "use strict"; return a + b; }

If the function should be in [strict mode](/en-US/docs/Web/JavaScript/Reference/Strict_mode), and the
entire script or enclosing function is also okay to be in strict mode, you can move the
`"use strict"` directive outside of the function:

```js example-good
"use strict";
function sum(a = 1, b = 2) {
  return a + b;
}

Function expression#

A function expression can use yet another workaround:

```js-nolint example-bad const sum = function sum([a, b]) { // SyntaxError: "use strict" not allowed in function with destructuring parameter "use strict"; return a + b; };

This can be converted to the following expression:

```js example-good
const sum = (function () {
  "use strict";
  return function sum([a, b]) {
    return a + b;
  };
})();

Arrow function#

If an arrow function needs to access the this variable, you can use the arrow function as the enclosing function:

```js-nolint example-bad const callback = (...args) => { // SyntaxError: "use strict" not allowed in function with rest parameter "use strict"; return this.run(args); };

This can be converted to the following expression:

```js example-good
const callback = (() => {
  "use strict";
  return (...args) => this.run(args);
})();

See also#

• {{jsxref("Strict_mode", "Strict mode", "", 1)}}
• {{jsxref("Statements/function", "function statement", "", 1)}}
• {{jsxref("Operators/function", "function expression", "", 1)}}
• {{jsxref("Functions/Default_parameters", "Default parameters", "", 1)}}
• {{jsxref("Functions/rest_parameters", "Rest parameters", "", 1)}}
• {{jsxref("Operators/Destructuring", "Destructuring parameters", "", 1)}}