Benchmark: test-string - MeasureThat.net

Tests:

array
String.prototype.replaceAt=function(index, char) { var a = this.split(""); a[index] = char; return a.join(""); } 'abcde'.replaceAt(2,'a')
obj.assign
String.prototype.replaceAt = Object.freeze(function(index, char) { return Object.assign([...this], {[index]: char}).join(``); }); 'abcde'.replaceAt(2,'a')

Rendered benchmark preparation results:

Previous results

Test case name	Result
array
obj.assign

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Latest run results:

Run details: (Test run date: 3 years ago)

User agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.138 Safari/537.36

Browser/OS: Chrome 81 on Windows 7

Test name	Executions per second
array	663231.1 Ops/sec
obj.assign	173069.9 Ops/sec

test-string (version: 0)