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)

SyntaxError.

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

• Default parameters
• Rest parameters
• Destructuring parameters

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

In this case, the functionsum has default parametersa=1 andb=2:

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 instrict mode, and theentire script or enclosing function is also okay to be in strict mode, you can move the"use strict" directive outside of the function:

"use strict";function sum(a = 1, b = 2) {  return a + b;}

A function expression can use yet another workaround:

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:

const sum = (function () {  "use strict";  return function sum([a, b]) {    return a + b;  };})();

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

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:

const callback = (() => {  "use strict";  return (...args) => this.run(args);})();

• Strict mode
• function statement
• function expression
• Default parameters
• Rest parameters
• Destructuring parameters