Variable used before definition JS-0129
Bug risk
Major
5 months ago
—
2 years old
's' was used before it was defined
113 : (s = r) === null || typeof s == "undefined"
114 ? "" + s
115 : {}.toString
116 .call(s) 117 .match(/\s([a-z]+)/i)[1]
118 .toLowerCase();
119 if (!new RegExp(o).test(a))'s' was used before it was defined
111 r && l.isElement(r)
112 ? "element"
113 : (s = r) === null || typeof s == "undefined"
114 ? "" + s 115 : {}.toString
116 .call(s)
117 .match(/\s([a-z]+)/i)[1]'t' was used before it was defined
386 t.zoomedHd.removeEventListener("load", g),
387 t.zoomedHd.classList.add("medium-zoom-image--opened"),
388 t.zoomedHd.addEventListener("click", r),
389 document.body.appendChild(t.zoomedHd), 390 u();
391 })))
392 : u();'r' was used before it was defined
385 (g = t.zoomedHd.addEventListener("load", function () {
386 t.zoomedHd.removeEventListener("load", g),
387 t.zoomedHd.classList.add("medium-zoom-image--opened"),
388 t.zoomedHd.addEventListener("click", r), 389 document.body.appendChild(t.zoomedHd),
390 u();
391 })))'t' was used before it was defined
385 (g = t.zoomedHd.addEventListener("load", function () {
386 t.zoomedHd.removeEventListener("load", g),
387 t.zoomedHd.classList.add("medium-zoom-image--opened"),
388 t.zoomedHd.addEventListener("click", r), 389 document.body.appendChild(t.zoomedHd),
390 u();
391 })))
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++;
}