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Sleep in NodeJS

Feb 8, 2022

One way to delay execution of a function in NodeJS is to use the seTimeout() function. Just put the code you want to delay in the callback. For example, below is how you can wait 1 second before executing some code.

setTimeout(function() {
  console.log('This printed after about 1 second');
}, 1000);

Using async/await

You can use async/await with promises to delay execution without callbacks.

function delay(time) {
  return new Promise(resolve => setTimeout(resolve, time));
} 

run();

async function run() {
  await delay(1000);
  console.log('This printed after about 1 second');
}

Using the Sleep Command

You can use execSync to invoke your OS' sleep command.

const {execSync} = require('child_process');

execSync('sleep 1'); // block process for 1 second.

This is different from using the delay(time) function from the previous examples because delay(time) is still non-blocking. For example, you can run multiple delay() calls in parallel using Promise.all() 

async function run() {
  const start = Date.now();
  await Promise.all([delay(1000), delay(1000)]);
  // Prints about 1000, because the `delay()` calls run in parallel
  console.log('Elapsed:', Date.now() - start);
}

function delay(time) {
  return new Promise(resolve => setTimeout(resolve, time));
}

run();

However, with execSync, you cannot run multiple execSync('sleep 1') in parallel. execSync() blocks the entire Node process, meaning no other code can execute. Be careful about using execSync()!

const {execSync} = require('child_process');

const start = Date.now();
execSync('sleep 1');
execSync('sleep 1');
// Prints about 2000, because `execSync()` runs in series
console.log('Elapsed:', Date.now() - start);
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