SyntaxError: "use strict" not allowed in function with "x" parameter

Message

SyntaxError: "use strict" not allowed in function with "x" parameter (Firefox)
SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list (Chrome)

Error type

SyntaxError.

What went wrong?

"use strict" directive appears at the top of a function that has one of the follow parameters.

• Default parameters
• Rest parameters
• Destructuring parameters

"use strict" is not allowed at the top of such function.

Examples

Function statement

In this case, the function "sum" has default parameters "a=1" and "b=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 in strict mode, and the entire script or enclosing function is also okay to be in strict mode, you can move "use strict" outside.

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

Function expression

Function expression can use yet another workaround.

var sum = function sum([a, b]) {
  // SyntaxError: "use strict" not allowed in function with destructuring parameter
  "use strict";
  return a + b;
};

This can be converted into following expression.

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

Arrow function

If it's an arrow function and needs to access this variable, you can use arrow function as the enclosing function.

var callback = (...args) => {
  // SyntaxError: "use strict" not allowed in function with rest parameter
  "use strict";
  return this.run(args);
};

This can be converted into following expression.

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

See also

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