gistfile1.txt

const AWS = require('aws-sdk');
const S3 = new AWS.S3({'region':'ap-northeast-1'});

exports.handler = async (event) => {
  let params = {
    'Bucket': 'バケット名',
    'Key': 'hoge/sample.json'
  }
  const data = await existFile(params)
}

async function existFile(params){
  let bool = true
  await S3.getObject(params).promise().catch(e=>{
    bool = false
  })
  return bool
}