Message
Firefox: SyntaxError: "use strict" not allowed in function with default parameter SyntaxError: "use strict" not allowed in function with rest parameter SyntaxError: "use strict" not allowed in function with destructuring parameter Chrome: SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list
Error type
What went wrong?
A "use strict" directive is written at the top of a function that has one of the following parameters:
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:
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 the "use strict" directive outside of the function:
'use strict';
function sum(a = 1, b = 2) {
return a + b;
}
Function expression
A 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 to the following expression:
var 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:
var 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:
var callback = (() => {
'use strict';
return (...args) => {
return this.run(args);
};
})();