improve keepNames behavior for profiling tools #4296
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In my esbuild app with keepNames: true, minify: false I noticed that for declarations `let foo = function() {}` the function shows in V8 profiles as anonymous. This is due to the transformation to `let foo = __name(function() {}, 'foo')` which prevents the NamedEvaluation on the anonymous function which typically happens natively (eg: https://tc39.es/ecma262/#sec-variable-statement-runtime-semantics-evaluation for VariableDeclaration).
Evidently V8 keeps an internal idea of the function name distinct from the .name property. To work around this, we transform these instead to `let foo = __firstValue({ foo: function() {} })` which takes advantage of the name assignment in https://tc39.es/ecma262/#sec-runtime-semantics-propertydefinitionevaluation. With this fix, these functions show names correctly in the Chrome profiler.
sophiebits
commented
Oct 6, 2025
| // Update the "name" property on the function or class for "--keep-names" | ||
| export var __name = (target, value) => __defProp(target, 'name', { value, configurable: true }) | ||
| export var __firstValue = (obj) => { for (var k in obj) { if (__hasOwnProp.call(obj, k)) return obj[k]; } } |
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(not sure if esbuild intends to support mutating Object.prototype; if not, can remove the __hasOwnProp check)
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@evanw I would appreciate a review here if you have some time 🙏 |
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In my esbuild app with keepNames: true, minify: false I noticed that for declarations
let foo = function() {}the function shows in V8 profiles as anonymous, whereas it works fine for keepNames: false, minify: false. This is due to the transformation tolet foo = __name(function() {}, 'foo')which prevents the NamedEvaluation on the anonymous function which typically happens natively (eg: https://tc39.es/ecma262/#sec-variable-statement-runtime-semantics-evaluation for VariableDeclaration).Evidently V8 keeps an internal idea of the function name distinct from the .name property. To work around this, we transform these instead to
let foo = __firstValue({ foo: function() {} })which takes advantage of the name assignment in https://tc39.es/ecma262/#sec-runtime-semantics-propertydefinitionevaluation. With this fix, these functions show names correctly in the Chrome profiler.