Variable used before definition JS-0129
Bug risk
Major
4 months ago
—
3 years old
'c' was used before it was defined
9089 )
9090 for (var l = 0; l < n.length; l++) {
9091 const h = (c = n[l])[0]
9092 const d = c[1] 9093 i[h] = new e.Vector(d)
9094 }
9095 for (l = 0; l < o.length; l++) {'c' was used before it was defined
9088 "'"
9089 )
9090 for (var l = 0; l < n.length; l++) {
9091 const h = (c = n[l])[0] 9092 const d = c[1]
9093 i[h] = new e.Vector(d)
9094 }'v' was used before it was defined
9014 }
9015 if (l.presence === e.Query.presence.REQUIRED) {
9016 for (m = 0; m < l.fields.length; m++) {
9017 o[(v = l.fields[m])] = o[v].intersect(h) 9018 }
9019 }
9020 }'v' was used before it was defined
9014 }
9015 if (l.presence === e.Query.presence.REQUIRED) {
9016 for (m = 0; m < l.fields.length; m++) {
9017 o[(v = l.fields[m])] = o[v].intersect(h) 9018 }
9019 }
9020 }'v' was used before it was defined
9007 }
9008 } else {
9009 void 0 === a[v] && (a[v] = e.Set.empty),
9010 (a[v] = a[v].union(L)) 9011 }
9012 }
9013 }
Description
It is recommended to use a variable only after it is defined as it might produce errors at runtime.
In JavaScript, prior to ES6, variable and function declarations are hoisted to the top of a scope, so it's possible to use identifiers before their formal declarations in code. This can be confusing and some believe it is best to always declare variables and functions before using them.
In ES6, block-level bindings (let and const) introduce a "temporal dead zone" where a ReferenceError will be thrown with any attempt to access the variable before its declaration.
Bad Practice
alert(a);
var a = 10;
f();
function f() {}
function g() {
return b;
}
var b = 1;
{
alert(c);
let c = 1;
}
Recommended
var a;
a = 10;
alert(a);
function f() {}
f(1);
var b = 1;
function g() {
return b;
}
{
let c;
c++;
}