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Added Symbol.toStringTag support to Promise #1421

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Apr 3, 2019
Merged

Added Symbol.toStringTag support to Promise #1421

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ae533fe
Added Symbol.toStringTag support to Promise
Jul 13, 2017
d6a3dc2
Returned Object for Symbol.toStringTag
Jun 13, 2018
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8 changes: 8 additions & 0 deletions src/promise.js
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Original file line number Diff line number Diff line change
Expand Up @@ -753,6 +753,14 @@ Promise.prototype._settledValue = function() {
// Implicit undefined for cancelled promise.
};

if (typeof Symbol !== "undefined" && Symbol.toStringTag) {
es5.defineProperty(Promise.prototype, Symbol.toStringTag, {
get: function () {
return "Promise";
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If the value returned is "Object" instead, we would get the exact same external toString.call behavior as we do now

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@spion spion Jun 13, 2018 •
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Additionally, libraries that use x[Symbol.toStringTag] === 'Promise' to differentiate native promises from Bluebird will continue to work normally. Only libraries that check for the mere presence of x[Symbol.toStringTag] will have problems.

Hmm. I see we'd still have an incompatible type though. Not sure what to do about this.

It looks like TS may have made an error here and effectively introduced a silly nominal type system.

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Wrote about it here: microsoft/TypeScript#19006 (comment)

}
});
}

function deferResolve(v) {this.promise._resolveCallback(v);}
function deferReject(v) {this.promise._rejectCallback(v, false);}

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