REPL tool on steroids
Last updated 4 months ago by benoitzohar .
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REPLx - A REPL CLI tool on steroids

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"Read-Eval-Print-Loop-times" allows you to run a Javascript code with node and monitor the execution time for as many executions as you want.


You may want to install replx globally to be able to use it wherever you want:

npm i -g @benoitzohar/replx


replx [options] <source> [times]

Run an inline script:

$ replx "console.log('hello')" 3
 [Inline code] 1.312974ms

or load a file:

$ replx myfile.js
 oh hello
 [myfile.js] 1.3433752ms

You can watch for file changes and rerun replx everytime:

$ replx --watch myfile.js


$ replx -w myfile.js


To compare multiple functions, you can create a file that exports as many function as you'd like and replx will compare the time for you. For example:

const base = "Lorem ipsum dolor sit amet, mea dolor placerat consectetuer ut";

module.exports.A = () => {
  const res = [];
  base.split().forEach(c => res.push(c.toUpperCase()));
  return res;

module.exports.B = () => {
  const res = base.split().map(c => c.toUpperCase());
  return res;

Run multiple times

Of course, one run is not enough to determine which function is faster, so you can easily tell replx to run you code multiple time. For example, here we will run the code for each functions in the file 1000000 times:

$ replx myfile.js 1000000

Note that the returned value of each function will not be logged if you run the code multiple times.


You can create an empty file automatically if it doesn't exist:

$ replx --create myfile.js


$ replx -c myfile.js

Since the file will be empty by default, we suggest that you use -c along with -w so you can use replx -c -w myfile.js and start measuring right away.

If you want to make a comparison, you can create a file with 2 exported functions:

$ replx --comparison myfile.js


$ replx -k myfile.js


$ replx -k -w myfile.js 1000
[B] 0.08004ms
[A] 0.098599ms (1x slower)
[A] 0.086088ms
[B] 0.166884ms (2x slower)

This command will:

  • create the file myfile.js with 2 exports functions:
module.exports.A = () => {
  return "A";

module.exports.B = () => {
  return "B";
  • open it in your default IDE (based on the file extension)
  • run the code 1000 times for each function
  • compare the time between the two functions
  • watch for file change and re-run, etc.


Use replx or replx -h for help.


For now, if you use replx as an inline script, be aware that the timing includes the cost of eval() at each loop. This means that the time to run displayed is not exactly the time to run your script.
In the same spirit, if you use a file, the cached cost of require() will be included.

However, using replx to compare the time-to-run of two different scripts will work as expected since the require cost will be the same in all the variants.

The run time can vary greatly between runs, we suggest that you run replx more than once to ensure the results are aligned.

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