javascript-callback-example.js

const assert = require('assert')

class Klass {
  // correct
  methodWithCall (val, cb) {
    this.val = val
    
    return cb.call(this, val)
  }

  // who is `this.cb`
  methodWithThis (val, cb) {
    return this.cb(val)
  }

  // `this` is undefined
  method (val, cb) {
    return cb(val)
  }
  
  /**
   * @param {string} val1
   */
  callback (val) {
    return this.val === val
  }
}

function main () {
  const k = new Klass()

  assert(k.methodWithCall(1, k.callback))

  try {
    k.methodWithThis(1, k.callback)
  } catch (e) {
    assert.strictEqual(e.message, 'this.cb is not a function')
  }
  
  try {
    k.method(1, k.callback)
  } catch (e) {
    assert.strictEqual(e.message, 'Cannot read property \'val\' of undefined')
  }
}

main()