Chapter 2. Implement program flow

Question 1

array.join method

Answer 1

Accepts a string as a parameter whichj is used as a delimiter to separate the values in the array

new Array('socer', 'Basketball','hockey').join(', ')
"socer, Basketball, hockey"

Question 2

array.reverse()

Answer 2

reverses the array

Question 3

array.slice()

Answer 3

var sports = new Array('soccer', 'basketball', 'hockey', 'football', 'cricket', 'rugby',
'tennis', 'badminton');
var someSports = sports.slice(1, 2);

The slice() method returns a shallow copy of a portion of an array into a new array object selected from begin to end (end not included).

Question 4

array.splice()

Answer 4

changes the contents of an array by removing or replacing existing elements and/or adding new elements in place.
splice(whereItShouldStart,numberOfItemsToSplice, itemsWhichReplace)
the optional parameter doesent need to be the same length

Question 5

array.shift() and array.unshift()

Answer 5

Just like Queue and Dequeue

Question 6

array.some()

Answer 6

checks whether any objects meet the criteria (callback)

var evenNumbers = new Array(0, 2, 4, 6, 8, 9, 10, 12);
var allEven = evenNumbers.some(evenNumberCheck, evenNumbers);
if (allEven) {
    ...
} else {
    ...
}
function evenNumberCheck(value, index, array) {
    return (value % 2) == 0;
}

Question 7

array.reduce() and array.reduceRight()

Answer 7

The reduce() method executes a reducer function (that you provide) on each member of the array resulting in a single output value.

The reduceRight goes from the right side of the array

Question 8

the break keyword in a loop

Answer 8

The break keyword breaks out of only the currently running loop. If the loop containing the break is nested inside another loop, the outer loop continues to iterate as controlled by its own expression.

Question 9

How can you hook up an event?

Answer 9

1. Declare it directly in the HTML markup
2. Assign the function to the event property of the element object through javascript
3. Use the newer add and remove methods on the element object to associate event handlers

Question 10

Exception object properties

Answer 10

1. message
2. number
3. Name

Question 11

var

Answer 11

Declares a variable, optionally initializing it to a value.

Question 12

let

Answer 12

declares a block-scoped, local variable, optionally initializing it to a value

Question 13

const

Answer 13

declares a block-scoped, read-only named constant

Question 14

arguments in Javascript functions

Answer 14

add(2, 3, 4); // 5
// added the first two; 4 was ignored

Question 15

arguments object (in a function)

Answer 15

Functions have an additional variable inside their body called agruments which is an array-like object hodling all the values passed into the function

function add() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum;
}

Question 16

array.length

Answer 16

array.length isnt necessary the number of items in the array.

var a = ['dog', 'cat', 'hen'];
a[100] = 'fox';
a.length; // 101

array.length is one more than the highest index in the array

Question 17

Objects - two ways of initializing an object

Answer 17

1. var obj = new Object();
2. var obj = {}

The second one is more convinient

Question 18

Rest parameter syntax

Answer 18

The rest parameter syntax allows us to represent an indefinite number of arguments as an array.

function avg(...args) {
  var sum = 0;
  for (let value of args) {
    sum += value;
  }
  return sum / args.length;
}

avg(2, 3, 4, 5); // 3.5

Question 19

Person.prototype

Answer 19

function Person(first, last) {
  this.first = first;
  this.last = last;
}
Person.prototype.fullName = function() {
  return this.first + ' ' + this.last;
};
Person.prototype.fullNameReversed = function() {
  return this.last + ', ' + this.first;
};

Person.prototype is an object shared by all instances of Person. Any time you attempt to access a property that isnt set, Javascript will check Person.prototype and it becomes avaliable to all instances of that constructor via the this object

Question 20

closure

Answer 20

Is the combination of a function and the scope object in which it was created.
Closures let you save state

Question 21

Number.isSafeInteger(testValue)

Answer 21

tests wether or not the value is safe

Question 22

Throwing a generic error

Answer 22

try {
  throw new Error('Whoops!');
} catch (e) {
  console.log(e.name + ': ' + e.message);
}

Question 23

Handling a specific error

Answer 23

If you’re writing for modern JavaScript engines, instanceof keyword:

try {
  foo.bar();
} catch (e) {
  if (e instanceof EvalError) {
    console.log(e.name + ': ' + e.message);
  } else if (e instanceof RangeError) {
    console.log(e.name + ': ' + e.message);
  }
  // ... etc
}

Question 24

geolocation - format

Answer 24

navigator.geolocation.getCurrentPosition(function(position) {
do_something(position.coords.latitude, position.coords.longitude);
});

Question 25

// global scope
var a = 1;

function one() {
  alert(a);
}

Answer 25

// alerts ‘1’

Question 26

calculate three()

// global scope
var a = 1;

function two(a) { 
  alert(a);
}

function three() {
  var a = 3;
  alert(a); 
}

Answer 26

// alerts ‘3’

Question 27

var a = 1;

function four() {
  if (true) {
    var a = 4;
  }

alert(a);
}

Answer 27

// alerts ‘4’, not the global value of ‘1’

Question 28

var a = 1;

function one() {
  if (true) {
    let a = 4;
  }

alert(a);
}

Answer 28

// alerts ‘1’ because the ‘let’ keyword uses block scoping

Question 29

var a = 1;

function Five() {
  this.a = 5;
}

alert(new Five().a);

Answer 29

// alerts ‘5’

Question 30

var a = 1;

var six = (function() {
  var a = 6;

  return function() {
    alert(a); 
  };
})();

Answer 30

var a = 1;

var six = (function() {
  var a = 6;

  return function() {
    // JavaScript "closure" means I have access to 'a' in here,
    // because it is defined in the function in which I was defined.
    alert(a); // alerts '6'
  };
})();

Question 31

var a = 1;

function seven() {
  this.a = 7;
}
seven.prototype.a = -1;

seven.prototype.b = 8;

alert(new seven().a); ?
alert(new seven().b); ?

Answer 31

var a = 1;

function seven() {
  this.a = 7;
}

// [object].prototype.property loses to
// [object].property in the lookup chain. For example...

// Won't get reached, because 'a' is set in the constructor above.
seven.prototype.a = -1;

// Will get reached, even though 'b' is NOT set in the constructor.
seven.prototype.b = 8;

alert(new seven().a); // alerts '7'
alert(new seven().b); // alerts '8'

Question 32

var x = 5;

(function () {
    console.log(x);
    var x = 10;
    console.log(x); 
})();

Answer 32

This will print out undefined and 10 rather than 5 and 10 since JavaScript always moves variable declarations (not initializations) to the top of the scope, making the code equivalent to:

var x = 5;

(function () {
    var x;
    console.log(x);
    x = 10;
    console.log(x); 
})();

Question 33

focus event bubbling

Answer 33

it does not bubble up unlike others

Question 34

event bubbling- event-target

Answer 34

The most deeply nested element that caused the event is called a target element, accessible as event.target.

Question 35

entry point function in javascript

Answer 35

window.onload = function(){…}

Question 36

array.every()

Answer 36

method lets you process specific logic for each array element to determine whether all of them meet some condition

Question 37

signature of array methods (handler)

Answer 37

array.filter(function(currentValue, index, arr), thisValue)

thisValue- what will be represented by .this in the handler

Question 38

canceling an event - format

Answer 38

window.onload = function () {
    var a = document.getElementById("aLink");
    a.onclick = OverrideAnchorClick;
}
function OverrideAnchorClick() {
    //do custom logic for the anchor
    window.event.returnValue = false;
    //or
    //return false;
}

Question 39

cancel bubble

Answer 39

window.event.cancelBubble = true;

Question 40

web socket -format

Answer 40

wsConnection = new WebSocket(‘wss://studygroup.70480.com’,[‘soap’,’xmpp’]);

Question 41

web worker postMessage

Answer 41

Starts the worker process. Single parameter which is data. If nothing is required empty string can be supplied

Question 42

web worker terminate

Answer 42

stops the worker process from continuing

Question 43

webWorker.onmessage = function(event){…}

Answer 43

After the worker process completes the onmessage function is called. This should be set up before starting the worker

Question 44

webworker self keyword

Answer 44

like .this for the web worker. Gives access to the global namespace within the web worker

Question 45

Using the eval function

Answer 45

The eval function is used to run JavaScript dynamically. It takes a string as a parameter and runs it as a JavaScript function.

Question 46

iFrame sandbox

Answer 46

The sandbox attribute should always be used to restrict what data can be placed into an iFrame

allow-same-origin - treated as being from the same Html document
allow-top-navigation - iFrame can load content from the containing HTML document
allow-forms iFrame can submit forms
allow-scripts iFrame can run scripts

Question 47

XMLHttpRequest

Answer 47

var xmlhttp = new XMLHttpRequest();
var url = "myTutorials.txt";

xmlhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
        var myArr = JSON.parse(this.responseText);
        myFunction(myArr);
    }
};
xmlhttp.open("GET", url, true);
xmlhttp.send();

Question 48

XMLHttpRequest.responseXML

Answer 48

XMLHttpRequest.responseXML read-only property returns a Document containing the HTML or XML retrieved by the request; or null if the request was unsuccessful, has not yet been sent, or if the data can’t be parsed as XML or HTML.

Question 49

XMLHttpRequest blob

Answer 49

var xReq = new XMLHttpRequest();
xReq.open("POST", "saveImage.aspx", false);
xReq.responseType = 'blob';
xReq.send(data);