local storage and performance

Question 1

How are cookies set and stored?

Answer 1

The server sends the cookie in the HTTP server response header as a name = value pair. The browser stores the cookie and sends it back in the request header in the same way.

Question 2

What is a way to get around the limitation on number of cookes that a domain can send?

Answer 2

Use subcookies. Rather than name = value, use name = name1=value1&name2=value2…

Question 3

How do you write a try/catch statement?

Answer 3

try { 
  if ( a ) { throw errorObject };
} catch (error) { // error arg is required
  doSomething( );
} finally { // optional }

Question 4

localStorage vs. IndexedDB

Answer 4

localStorage is synchronous and string only storage, intended mainly for smaller amounts of data. It will block page loading.
IndexedDB is more database-like; asynchronous, for greater amounts of data

Question 5

How is an object represented in json (4 key elements) ?

Answer 5

Like a javascript object, but with a few key differences: key must be in quotes; no comma after last pair; no trailing semicolon; must use double quotes.

Question 6

When is try-catch statement most useful?

Answer 6

For handling an error that is out of your control —for example, when dealing with a library whose code you can’t touch.

Question 7

When should you catch errors vs. when should you throw errors using throw new Error( )?

Answer 7

Catch errors only if you know what to do — you want to prevent browser from doing its default thing.
Throw errors if you don’t know what to do. Purpose is to provide information about why error occurred.

Question 8

What is the most bomber way to do type detection (e.g. to determine if a value is an array or function or regexp or whatever)?

Answer 8

Object.prototype.toString.call ( valueToTest );

This will return a string in format of “[Object NativeConstructorName “]; // e.g. [Object Array]

Question 9

JSON allows representation of what types of values?

Answer 9

Simple values, objects (string-value mappings), and arrays. It doesn’t more complex data types like functions or regular expressions.

Question 10

What are the JSON object’s methods? And what do they do?

Answer 10

JSON.stringify(someObj) serializes an object into a JSON string.
JSON.parse(someJSON) turns a JSON string into a javascript value.

Question 11

What arguments does JSON.stringify() take?

Answer 11

The object to serialize; a filter, which can be an array or a function; indentation amount.

Question 12

What is the execution context in javascript?

It can be thought of as an object with three properties – what are they?

Answer 12

An execution context is created every time a function is called (there’s also one for evals, withs, and global). 1. variable object; 2. scope chain; 3. ‘this’ object.

Question 13

How can you filter what data gets serialized into a JSON string?

Answer 13

The second stringify argument can be an array of keys (e.g. [“title”, “author”]). Only those keys will be serialized.
Or it can be a function that can be more nuanced or allow you to make changes.

Question 14

What is ECMA5 definition of a property?

Answer 14

An association between a name and a value that is part of an object
In other words, part of an object.

Question 15

What is ECMA5 definition of a variable?

Answer 15

There isn’t one. But it seems to be something like this:

An association between a name and a value that is part of an execution context. It belongs to a VariableObject.

Question 16

Why are properties and variables sometimes interchangeable? E.g.var foo = “bar”; log(window.foo); and window.bar = “foo”; log(bar).

Answer 16

This only works because the global object (parent of properties) and the global VariableObject (parent of variables) happen to be the same. In the function context, of course, property/variable switching will fail.

Question 17

Code can be executed within what types of execution context?

Answer 17

global (only one); functional (any time a function is called); eval;

Question 18

What is the activation object / variable object?

Answer 18

Every execution context has an activation object, which contains all the variables and functions that are in its scope, plus the arguments object, plus the args. For functions, the activation object is called a variable object.

Question 19

Why is it important to avoid global lookups?

Answer 19

Because global variables and functions involve a scope chain lookup, which is expensive. example : cache reference as in, var doc = document;

Question 20

What is a multiple property lookup?

Answer 20

window.location.href.substring(window.location.href.indexOf(“?”));
Expensive. If used more than once, store in a local variable. var href = window.location.href;

Question 21

What are four steps to optimizing a loop?

Answer 21

Decrement iterators are more efficient. (start at max and go to 0);
Since it must be evaluated every iteration, simplify terminal condition as much as possible. Avoid property lookups.
Simplify loop body.
Use posttest loops (do/while) because they avoid the initial evaluation.

Question 22

It’s more performant to have fewer statements. What are some ways to reduce the number of statements?

Answer 22

Declare a bunch of variables at once with commas;
Make an object or array using literals rather than multiple statements.
Combine iterative values into statement ( e.g. name = values[ i++ ] )