Is there any difference between declared and defined variable答案

【问题标题】：Is there any difference between declared and defined variableIs there any difference between declared and defined variable
【发布时间】：2020-02-28 09:36:58
【问题描述】：

I try writing the following lines in the console one by one

let x = y //throws error "Uncaught ReferenceError: y is not defined"
console.log(x) //throws error "ReferenceError: x is not defined"
let x = 3; //gives error "Uncaught SyntaxError: Identifier 'x' has already been declared"
x = 3 //ReferenceError: x is not defined

Now problem is that how can be a variablenot definedandhas been declaredat the same time. Is there any difference between both.

【问题讨论】：

It's a poor use of language by the browser consoles. Those ReferenceErrors should really say the variable is not "declared", but afaik it's always been thus.
From link:When there's assignment, the right-hand side is parsed first; if the right-hand side throws an error, it never gets to the left-hand side, and the variable declared with let never gets properly initialized; it'll stay in the demilitarized zone / temporal dead zone forever(and you can't re-declare a variable that's already been declared, even though the attempted assignment during initialization threw an error).
There's a big difference betweendeclarationandinitialization. In your first line x = y, you declared xand tried to assign y to it, which is undefined, so xis declared and will be initialized with undefined as value. That's why you got x is already declared.
@MaheerAli let x = 3; will throw error as you have already declared it. But x=3 should not throw error, unless you define it as const
Being declared doesn't mean it will be initialized to undefined. Are variables declared with let or const not hoisted in ES6?

标签： javascript variables

【解决方案1】：

A let or const variable can only bedeclaredonce - that is, when you have let <variableName> in a scope, you have declared <variableName> in that scope, and cannot declare it again in that scope.

From the previously linked question:

When there's assignment, the right-hand side is parsed first; if the right-hand side throws an error, it never gets to the left-hand side, and the variable declared with let never gets properly initialized; it'll stay in the demilitarized zone / temporal dead zone forever

You can't re-declare a variable that's already been declared, even though the attempted assignment during initialization threw an error.
But on line 4, x=3 should do a proper assignment and it should remove x from TDZ. But that also fails. I fail to understand that

After a variable has beeninitialized(for example, the let x runs), it can be assigned to. But just like you can't assign to a variablebeforeits let initialization, you also can't assign to a variable later, when its initialization did not complete successfully:
x = 'foo';
let x = 'bar';
Error:
Uncaught ReferenceError: x is not defined

Which is the same sort of thing that happens in the console when you try:
let x = y
// Uncaught ReferenceError: y is not defined
// x has not been initialized, so the next line throws:
x = 'foo'
// Uncaught ReferenceError: x is not defined
x still has not been initialized, so the error is the same.

Encountering this sort of thing ispretty odd, though - you only see it in the console. In normal scripts, a thrown error will prevent further execution, and the fact that a variable name remains uninitialized forever is not something to worry about.

The above was an issue in earlier Chrome versions. But in Chrome 80+, re-declarations of let are now permitted, so the error

Uncaught SyntaxError: Identifier 'x' has already been declared

should no longer occur, regardless of whether the previous initialization of the variable succeeded or not:

【讨论】：

I'd like to point out that in Firefox's console let x = y does initialize x and x = "foo" doesn't throw. However doing the same out of the console, it throws correctly.