Found unsafe function declarations JS-0073
Bug risk
Major
4 months ago
—
3 years old
Function declared in a loop contains unsafe references to variable(s) 'r', 'g', 'r', 'r'
4018 : (v = Q.access(e, 'fxshow', { display: l })),
4019 o && (v.hidden = !g),
4020 g && fe([e], !0),
4021 p.done(function () { 4022 for (r in (g || fe([e]), Q.remove(e, 'fxshow'), d)) { 4023 k.style(e, r, d[r]) 4024 } 4025 })), 4026 (u = pt(g ? v[r] : 0, r, p)),
4027 r in v ||
4028 ((v[r] = u.start), g && ((u.end = u.start), (u.start = 0)))
Description
Writing functions within loops tends to result in errors due to the way the function creates a closure around the loop. For example:
for (var i = 0; i < 10; i++)
funcs[i] = function() {
return i;
};
}
In this case, you would expect each function created within the loop to return a different number. In reality, each function returns 10, because that was the last value of i in the scope.
let or const mitigate this problem.
for (let i = 0; i < 10; i++) {
funcs[i] = function() {
return i;
};
}
In this case, each function created within the loop returns a different number as expected.
Bad Practice
for (let i = 10; i; i--) {
(function() { return i; })();
}
while(i) {
const a = function() { return i; };
a();
}
do {
function a() { return i; };
a();
} while (i);
let foo = 0;
for (let i = 0; i < 10; ++i) {
//Bad, `foo` is not in the loop-block's scope and `foo` is modified in/after the loop
setTimeout(() => console.log(foo));
foo += 1;
}
for (let i = 0; i < 10; ++i) {
//Bad, `foo` is not in the loop-block's scope and `foo` is modified in/after the loop
setTimeout(() => console.log(foo));
}
foo = 100;
Recommended
let a = function() {};
for (let i = 10; i; i--) {
a();
}
for (let i = 10; i; i--) {
const a = function() {}; // OK, no references to variables in the outer scopes.
a();
}
for (let i = 10; i; i--) {
const a = function() { return i; }; // OK, all references are referring to block scoped variables in the loop.
a();
}
const foo = 100;
for (let i = 10; i; i--) {
const a = function() { return foo; }; // OK, all references are referring to never modified variables.
a();
}
//... no modifications of foo after this loop ...