Using const/let instead of var can make JavaScript code run 10× slower in Webkit

Re:Using const/let instead of var can make JavaScript code r... by judge2020 on Wednesday October 21, 2020 @01:22AM (#24844971)

If it was written like

  while true {
    let hello = "world";
  }

The scope would keep getting freed then re-allocated, which will be slower than simply continuously setting an existing variable's memory location over and over again.

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Re:Using const/let instead of var can make JavaScript code r... by charliesome on Wednesday October 21, 2020 @02:00AM (#24845143)

While the creation and destruction of a scope every loop iteration is the correct mental model for what this code does, there's no reason that implementations must actually perform any allocations - especially for an example like this where it is trivially determinable that no bindings escape the immediate scope.

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Re:Using const/let instead of var can make JavaScript code r... by chrismorgan on Wednesday October 21, 2020 @02:24AM (#24845245)

That would apply to a pure interpreter, but all JS runtimes will do at least some optimisations, even on their baseline profile.

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Re:Using const/let instead of var can make JavaScript code r... by ihuman on Wednesday October 21, 2020 @01:37AM (#24845042)

If that was the reason, then shouldn't "const" be as slow as "let"?

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Re:Using const/let instead of var can make JavaScript code r... by kjksf on Wednesday October 21, 2020 @02:08AM (#24845181)

It should be valid to move `const` outside of the loop, since by definition it cannot be changed.

`let` can be changed so cannot be optimized out this way (at least not without doing more complex analysis and proving that the variable is not being changed).

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Re:Using const/let instead of var can make JavaScript code r... by cosmosgenius on Wednesday October 21, 2020 @07:39PM (#24853499)

let wasn't inside the loop, which is what made it weird.

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