Functions and Scope

Question 1

What are two ways to tell if something called myVar is an array?

Answer 1

1. myVar instanceof Array;

2. Array.isArray(myVar); // not just isArray(var);

Question 2

What does the toFixed() method do? (and on what type of value?)

Answer 2

It’s a method of the Number type and it returns a string with a specified number of decimal places. if n = 10.0092, n.toFixed(2) will return “10.01”;

Question 3

var obj = new Object();
if (obj) { // will this take place? }

Answer 3

Yes, because objects convert to true when evaluated in Boolean expressions.

Question 4

What is a function declaration compared to a function expression?

Answer 4

declaration: function myFunc(){ … }
expression: var myFunc = function() { … }

Question 5

How could you get today’s day (e.g. “Monday”) from the date object?

Answer 5

You need to make an array with Sunday at 0 index and Saturday at 7 index.
Then use getDay() and find the day using your array.

Question 6

Name the two RexEx methods that work with patterns:

Answer 6

exec() and test()

Question 7

What is function declaration hoisting?

Answer 7

Before code begins running, the javascript engine “hoists” functions to the top of the execution context. That way, a function can be called even if it doesn’t appear in code until later. This only works with function declarations.

Question 8

var arr1 = [1,2,3];
var arr2 = arr1;
arr1 = null;
what happens to arr2?

Answer 8

arr2 = [1,2,3];
arr1 and arr2 were both pointers to the same reference object (an array) but arr1 had it’s relationship that array broken.

Question 9

function f1(){ // do something; }
var f2 = f1;
f1 = null;
What happens if you call f2( ); ?

Answer 9

the original function will run.
When you set f1 to null, you broke the relationship between the pointer and the function but you didn’t change the function.

Question 10

What does a for … in loop do and what does it look like?

Answer 10

It iterates over the enumerable properties in an object like this:
for (var prop in obj) { 
    // optionally, find only properties of this object, not the objects from which it inherits
    if (obj.hasOwnProperty(prop) { // do something

Question 11

How do you access the function you are in (e.g. for a factorial)?

Answer 11

Use the name of the function. Arguments.caller is deprecated.

Question 12

What are the string regular expression methods?

Answer 12

match()
search();
replace();
split();

Question 13

Describe the apply() vs call() function

Answer 13

Both take two arguments, the value of “this” and some optional arguments to be acted upon. Apply takes an arguments object or an array; call takes a list of arguments: someFunc.call(this, arg1, arg2, etc)

Question 14

How can a primitive value like “cats” can have access to methods, e.g. “cats”.substring(1,3) or “cats”.split(‘’);

Answer 14

Every time a primitive value is read, an object of the corresponding primitive wrapper type is created. This wrapper allows access to methods like substring() but it then is immediately destroyed.

Question 15

var s = String;
typeof s will be what?

Answer 15

“function”

Question 16

string.replace() method take what argument(s)?

Answer 16

the pattern to match against and the thing to replace it with. This second argument (the replacement text) can be a function which will apply further logic.

Question 17

var s = new String();
typeof s will be what?

Answer 17

“object”

Question 18

What is format for string matching methods?

Answer 18

stringName.match(regExPattern);

stringName.search(regExPattern);

Question 19

What are two ways to access the Global Object?

Answer 19

1. Use window (window.property or window.method);

2. Return it from an anonymous function: global = function(){ return this; }();

Question 20

how do you find a random number between 1 and 10?

Answer 20

var rand = Math.floor(Math.random() * 10 + 1);

Question 21

Can you immediately invoke a function declaration as well as a function expression?

Answer 21

You have to wrap the function in parentheses and add () at the end:
(function(){ // do something }()). This works because parentheses can’t contain statements, so the parser knows to treat it like an expression.

Question 22

use apply() to apply cat.getInfo() method to dog object.

Answer 22

cat.getInfo.apply(dog);

This will run cat’s getInfo() method but will do it using dog as the “this” object.

Question 23

How does the bind() function work?

Answer 23

var bound = nameOfFunction.bind(nameOfObj);
You need to tie it to a variable like this or the binding just gets lost. You can now say:
bound(); // it will call nameOfFunction and the object we passed will be the "this" object.

Question 24

When should you use typeof rather than instanceof operator?

Answer 24

Use typeof for testing simple build-in types, like strings, booleans, and numbers:
99.99 instanceof Number; // false
typeof 99.99 == ‘number’; //true
Also for functions and null.

Question 25

When should you use instanceof rather than typeof operator?

Answer 25

Use if for complex built-in types such as RegEx, Array, Object. Also use for custom types:
var ClassFirst = function () {};
var ClassSecond = function () {};
var instance = new ClassFirst();
typeof instance == 'ClassFirst'; // false
instance instanceof Object; //true
instance instanceof ClassFirst; //true

Question 26

for (var i=0; i<10; i++){
 // what is i here? 
}
// what is i here?
Why does this happen?

Answer 26

i will be 9 in the first place and 10 in the second. Javascript holds onto variables that are set in for loops.
This happens because JS doesn’t have block-level scope, only function and global scope.

Question 27

What is an anonymous function?

Answer 27

A function that isn’t bound to an identifier. In js terms, a function without a name after the function keyword as in var f = function(){ }; // anonymous

Question 28

What is a closure?

Answer 28

A closure is a function that has access to variables from another function’s scope —usually because it’s inside of the other function.

Question 29

What is an execution context and what are it’s three forms?

Answer 29

An execution context is the environment / scope the current code is being evaluated in. There is a global one, an infinite number of contexts for functions, and the context created by eval().

Question 30

What is the Execution Context Stack?

Answer 30

Only one thing can happen at a time in javascript, so the interpreter puts execution contexts in a stack and works through them, top to bottom (LIFO).

Question 31

A function that is defined inside another function adds what into its scope chain?

Answer 31

The containing object’s activation object.

Question 32

What is an activation object?

Answer 32

It’s an object that holds variables that are local to a given execution context.

Question 33

A closure can still use variables that are local to the containing function, even if the containing function is set to null. How is this possible?

Answer 33

Because the closure has a reference to the containing function’s activation object, which won’t be destroyed (because of the closure).

Question 34

Does a closure store a reference to the containing function’s variables directly?

Answer 34

No, it stores a reference to the entire variable object. If the container has a loop that iterates over i, the closure will only point to the last value of i, even if created by one of the loop’s earlier iterations.

Question 35

Can an inner function ever have direct access to the “this” or “arguments” variables of a containing function?

Answer 35

No, you need to save a reference to either of these in a separate variable, such as that = this;

Question 36

Are function arguments passed by reference or by value?

Answer 36

They are only passed by value. You can use this with closures to “fix” the value of variables in the containing function.

Question 37

What does “this” point to when inside an anonymous function?

Answer 37

inner functions never have access to an outer function’s this or arguments variables, so it points to window. Use that or self to deal with this.

Question 38

What happens if you declare a variable more than once? (e.g. var i = 10; var i; // i is now what?)

Answer 38

Javascript ignores declarations on variables that have already been declared. i will be 10.

Question 39

var x = window.y;
if y is undeclared, what is the value of x?

Answer 39

undefined. Javascript doesn’t throw an error because y is part of a property lookup.

Question 40

// global scope
var name = "Bill;
delete name; 
console.log(name); // what happens?

Answer 40

Name still exists as “Bill” because you can’t delete properties of window that are declared using var keyword.

Question 41

What is the way to create a timer?

Answer 41

var timeoutID = setTimeout( function(){
    // do something
}, 1000); //use an id to reference the timeout.

Question 42

How can you end a timer?

Answer 42

clearTimeout(timeoutID); // you need a reference to the timeout ID.

Question 43

What scope do the timer methods belong to?

Answer 43

It belongs to window. “This” refers to the global scope, window.