javascript - Weird Symbol problem

Question

These are the two lines of code extracted var name = Symbol('test') It keeps prompting that it cannot be converted. Is the keyword reserved? Or other reasons?
Why can it be compiled by changing var name1 = Symbol('test')?
Other ordinary var s1 s2 can also be compiled.

Answer 1

This question is quite interesting. I have never noticed it. After looking through the information, I found that many things came together by coincidence, and then this problem appeared. To be precise, it’s the browser’s default behaviorand JavaScript’s implicit type conversionthat’s causing the trouble.

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Let’s go step by step, first of all, what is the difference between var and let?

The variable declared by

var will be promoted to the top of the current function scope. If it is global, then this variable will become an attribute of window.
For variables declared by let, it will promote the variable to the current block-level scope, and if it is global, the current variable will not become an attribute of window.

So, in the big picture something like this happens:

var test1 = 'test1';
let test2 = 'test2';

console.log(window.test1);    // test1
console.log(window.test2);    // undefined

Then, what is the difference between the variable named name and other variables?
We know above that var name = 'test1'; can actually be equivalent to window.name = 'test1'. It is easy to think, is name a fixed reserved word?

Looking through the specifications, it’s true. The
window.name attribute represents the name of the current window context. The following are some interfaces of window:

[ReplaceableNamedProperties] 
interface Window {
  // the current browsing context
  readonly attribute WindowProxy window;
  readonly attribute WindowProxy self;
  readonly attribute Document document;
           attribute DOMString name;
  //..
}

The

name attribute in the last line here does not have the prefix readonly, indicating that it is readable and writable, and its data type is DOMString.
DOMString refers to a UTF-16 string, which is directly mapped to String in JavaScript.

So when we assign a value to window.name, this value will be coerced into String.

We can give it a try:

var name = { a:1, b:2 };
console.log(window.name);    // [object Object]

var name = [0, 1, 2, 3];
console.log(window.name);    // 0,1,2,3

You can probably guess at this point that the error in var name = Symbol('test'); should be that there is a problem with the type conversion of the Symbol variable. The actual error reported also confirmed our guess: TypeError: Cannot convert a Symbol value to a string.

However, it doesn’t seem right. Symbol variables can be converted into strings, for example:

let test = Symbol('test');

console.log(test.toString());    // Symbol(test)
console.log(String(test));       // Symbol(test)

Well, this is a cliché. JavaScript’s implicit type conversion and explicit cast are different for some variables. Unfortunately, Symbol falls into this category here.

When

Symbol is implicitly converted, it will first call its internal ToPrimitive interface to get its original value, and then call the ToString function to convert it to a string. Note that the ToString function here is its internal abstract method, and is not the same thing as the exposed Symbol.prototype.toString().

For the Symbol variable, an error will be reported when it calls ToString. I will not go into more details. If you are interested, you can read the specification for yourself: ToString (argument).

Answer 2

I just tried it on the console, it is indeed a very amazing BUG, ​​but you will

var name = Symbol("test"); 
//改成
let name = Symbol("test"); //试试。。

Then I was surprised to find that the BUG was gone again. . I guess it has something to do with how the browser parses syntax, but I don't understand these things.

Answer 3

name is a unique attribute of window. If you try changing the variable, you will not get an error. . .

Answer 4

name is a unique attribute of window. Any value assigned to the name variable in the global environment will be automatically converted into a string. However, the Symbol type cannot be directly converted into a string, so an error is reported.

You can

var name = 1;
console.log(name);

You’ll know.