Cannot convert a Symbol value to a string

[unional]

tsc: 2.1.0-dev.20161110

function foo(x: symbol) {
    console.log(`abc ${x}`)
}

transpile to:

function foo(x) {
    console.log("abc " + x);
}

results in "Cannot convert a Symbol value to a string" error at runtime.

http://www.typescriptlang.org/play/#src=function%20foo(x%3A%20Symbol)%20%7B%0D%0A%20%20%20%20console.log(%60abc%20%24%7Bx%7D%60)%0D%0A%7D

[Jessidhia]

Symbol values indeed can't be converted to any other type (other than by calling .toString()), and the type checker does catch other match-ups such as with number or Object, but it allows string...

[unional]

Yes, but in this case, as it is used in literal string, I would suggest to transpile it to .toString():

function foo(x) {
  console.log("abc " + x.toString())
}

The question is what to do when x: symbol | SomeOtherType.

[blakeembrey]

@unional I don't think TypeScript should change the function call implicitly. If anything, I think this should generate a TypeScript error for non-stringable entities like symbol.

Side note: What value is Symbol? I only know of symbol.

[unional]

I can see that it may not make sense to transform it to symbol.toString(), as described here:

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Template_literals

The default function just concatenates the parts into a single string.

I have just tried on the chrome devTools and indeed var a = Symbol(); abc ${a}`` yield the same error.

By the way, is there any other "non-stringable" entities?

EDIT: Symbolis the function while symbol is the type. Fixed OP

[blakeembrey]

By the way, is there any other "non-stringable" entities?

Not much comes to mind, but there's Object.create(null).

[unional]

If that's the case, would it be an overlooked case on JavaScript template literal side? Because:

var a = Symbol('a');
console.log(a); // "Symbol(a)"
console.log(`abc ${a}`); // TypeError: Cannot convert a Symbol value to a string

[blakeembrey]

No, you can do String(Symbol('a')) to convert it to a string (and console.log probably has specific visual implementations of every type anyway). JavaScript's coercion rules can be a little confusing.

[unional]

Yeah, I re-read the doc and here it is:

Symbol("foo") + "bar" throws a TypeError (can't convert symbol to string). This prevents you from silently creating a new string property name from a symbol, for example.
The "safer" String(sym) conversion works like a call to Symbol.prototype.toString() with symbols, but note that new String(sym) will throw.

var a = Symbol('a');
`abc ${String(a)}` // "abc Symbol(a)"
`abc $(new String(a)}` // TypeError: Cannot convert a Symbol value to a string

😞