javascript.md

Javascript

The problems with var

1. It is not scope safe, so you can accidentally overwrite variables in the outer scope:

Example 1:

var a = "a"
if (true) { var a = 2 }
console.log(a)  // 2

Example 2:

var i = 13
for (var i=1; i<4; i++) { }
console.log(i)  // 13

2. can use variables before they've been declared (hoisting #1):

   console.log(a) // prints "undefined", instead of an exception var a = 2

3. can use variables declared from an inner scope (hoisting #2:

   { var a = "2" } console.log(a) // 2

4. var variables are added to window object

Debugging tip

To print variables on console.

Instead of:

> console.log(`a = ${a}`)
a = 1

Wrap the variable in an object:

> console.log({a})
{ a: 1 }

arrow functions

const functionName = param => { body; };
const functionName = (param1, param2) => { body; };      

... syntax (spread & rest)

On RHS of assigment (spread expands the elements):

const other = [ 1, 2, ...rows ];    # expand all elements of 'rows' array

const dateFields = [1970, 0, 1];
const d = new Date(...dateFields);  # call Date() with array elements as params

# with objects:
const obj1 = { foo: 'bar', x: 42 };
const obj2 = { foo: 'baz', y: 13 };
const clonedObj = { ...obj1 };          # Object { foo: "bar", x: 42 }
const mergedObj = { ...obj1, ...obj2 }; # Object { foo: "baz", x: 42, y: 13 }

On LHS of assignment (spread collects the elements):

const [a, ...data] = rows;      # collector: put first (of 'rows' array) in 'a' and put rest in new 'data' array
const [, ...data] = rows;       # collector: as above but skip first

In function declaration (rest):

function sum( first, ...others ) { body; }  # collects rest of params in array 'others'

Async code

• Use AWS SDK .promise()

    const AWS = require('aws-sdk')
    const Lambda = new AWS.Lambda()
    await Lambda.invoke(req).promise()
  

• Use promisify()

    const fs = require('fs')
    const { promisify } = require('util')
    const readFile = promisify(fs.readFile)
  

• Avoid await inside forEach()

• No need for callbacks in AWS event handler, e.g.

    module.exports.handler = async (event, context) => {
      const response = {
        statusCode: 200,
        body: JSON.stringify({ message: 'hello world' })
      }
  
      return response
    }
  

Misc

eslint --print-config file.js
npm pack          # build local tar file
npm install {tarball}

Node search path

module.paths      # contains search path

Require with globally installed modules

By default require() doesn't search in global modules install location. To require() globally installed modules, either use npm link or NODE_PATH. E.g. export NODE_PATH=/usr/local/lib/node_modules

Hash

import crypto from 'crypto'
const data = crypto.createHash('sha256').update(s).digest()

Match

const match = line.match(/(...)=(\S+) -> (.*)/)
const [_, prefix, name, targetsText] = match!