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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
What went wrong?
"use strict" directive appears at the top of a function that has one of the follow 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);
};
})();