1 CS480W Quiz 6 Solution

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1 1 CS480W Quiz 6 Solution Date: Fri Dec Max Points: 15 Important Reminder As per the course Academic Honesty Statement, cheating of any kind will minimally result in receiving an F letter grade for the entire course. Please ensure that you ll-in your name in the bubbles on the provided grid-sheet. For each of the following questions, select a single alternative on the grid-sheet. There are 7 questions with 2-points per question; there is 1-point for submitting the quiz. 1. Which one of the following statements is always false? (a) If p is a Promise, then p.then(fn) will return a Promise wrapping the result of calling fn(). (b) A promise chain like p.then(handler1).then(handler2).then(handler3); will create 3 new promises as the return value of each call to then(). (c) A promise is created using the constructor call: new Promise(executor) where executor is a 2-argument function executor(resolve, reject), with resolve and reject themselves being 1-argument functions. (d) If p is a Promise, then p.catch(fn) will return a Promise wrapping the result of calling fn(). (e) If p is a Promise, then p.then(fn) will return whatever the call fn() returns. Answer: (e). (e) is false. p.then(fn) returns a Promise which when settled contains the value returned or exception thrown by fn(). (a) is true. p.then(fn) returns a Promise which when settled contains the value returned or exception thrown by fn(). (b) is true. Calling then() always returns a new Promise. 1

2 (c) is true. The Promise constructor takes a single executor function argument.. (d) is true. p.catch(fn) returns a Promise which when settled contains the value returned or exception thrown by fn(). 2. Given the following code: async function f(val, a, b) { return await Promise.resolve(val). then((v) => { throw v + b }). then((v) => { throw v + b }). catch((err) => err * b). then((v) => v + a); } what will be the result of calling await f(5, 3, 2)? (a) 17 (b) 27 (c) 14 (d) 23 (e) 29 Answer: (a). The promise-chain within f() is resolved as follows: (a) The Promise.resolve(val) initializes the promise-chain with 5. (b) The resolve() for the rst then() is called with value 5. It throws an error object with value (5 + 2) which is 7. (c) Because of the exception thrown by the rst then() the second then () is skipped with the error object remaining at 7. (d) Since the rst then() threw an error object 7, the catch() is entered with err set to 7. It returns (7 * 2). Hence the promise-chain value is set to 14. (e) The resolve() for the last then() is called with promise-chain value of 14. It resolves to (14 + 3). Hence the return value of the promisechain is 17. So the function returns Which one of the following CSS selectors selects the element having an id attribute quicksort which is a descendant of the <div> HTML element with class attribute algorithm? 2

3 (a) div.algorithm + #quicksort. (b) div.algorithm ~ #quicksort. (c) div.algorithm #quicksort. (d) div.algorithm > #quicksort. (e) None of the above Answer: (c). The selector div.algorithm selects all HTML elements with tag <div> having a class attribute algorithm. The space between that selector and the following #quicksort selector establishes the div.algorithm selector as the context within which a descendant element with id attribute #quicksort is selected. 4. Which one of the following statements is always false? (a) If an event handler for a react component needs to refer to the component, then it is necessary to ensure that this is bound to the component before the handler is invoked. (b) The names of all user-dened react components must start with an upper-case character. (c) The react implementation takes care to not re-render components unless absolutely necessary. (d) If the state of a react component needs to be updated, then the update must be made using the component's setstate() method. (e) When the state of a react component needs to be initialized from a web service, the state should be initialized by calling the web service in the component's constructor. Answer: (e). (e) is false. The web service call is likely to be asynchronous and asynchronous calls should not be made from within a constructor. Such initialization should be deferred to the componentdidmount() lifecycle method. (a) is true. A common idiom for ensuring this is to explicitly bind the handler to the component in the component constructor using code like this.handler = this.handler.bind(this); (b) is true. The names of all user-dened react components must start with an upper-case character so that react can distinguish them from components corresponding to HTML elements. 3

4 (c) is true. The react implementation uses a virtual-dom to ensure minimal re-rendering. This is necessary to obtain reasonable performance in apps with many 1000's of components. (d) is true. All state changes must be made using the setstate( ) method. This ensures that react is informed about state changes so that it can decide whether it needs to re-render the component. 5. Which one of the following statements is always false? (a) The Accept-Charset: header is a content negotiation header used by a client to specify the character sets it understands. (b) The Accept-Charset: header is used to specify the character set of text content within a HTTP request. (c) The Accept-Language: header is a content negotiation header used by a client to specify the human language and locale variant it prefers. (d) The Accept: header is a content negotiation header used by a client to specify the MIME types it understands. (e) The Content-Type: header is used to specify the character set of text content within a HTTP request. Answer: (b). (b) is false. The Accept-Charset: header species the character set which can be used in the response. It is the Content-Type: header which is used to specify the character set; it has a primary value which is a MIME-TYPE, but can also have a charset= sux which describes the character set used by text content in the request. (a) is true. The Accept-Charset: header species the character sets which can be understood by the client. It's value consists of commaseparated character sets with each character set suxed with an optional q-factor weight preceeded by a ;. An example header is: Accept-Charset: utf-8, iso ;q=0.5 (c) is true. The Accept-Language: header species the human language which can be understood by the client. It's value consists of commaseparated 2 or 3 character language specication followed optionally by a - followed by a locale specication. This language-locale may optionally be followed by a q-factor weight preceeded by a ;. An example header is: Accept-Language: en-us,en;q=0.5 (d) is true. The Accept: header species the content MIME types which can be understood by the client. It's value consists of comma-separated MIME types with each MIME type suxed with an optional q-factor weight preceeded by a ;. An example header is: 4

5 Accept: text/html, application/xml;q=0.9, */*;q=0.8 (e) is true. The Content-Type: header is used to specify the character set; it has a primary value which is a MIME-TYPE, but can also have a charset= sux which describes the character set used by text content in the request. 6. Which one of the following statements is always true? (a) If a Set-Cookie: header species a cookie having the HttpOnly attribute, then that cookie will not be sent for HTTPS requests. (b) If a Set-Cookie: header species a cookie having the Secure attribute, then that cookie can only be stored in encrypted storage. (c) A cookie is sent from the client to the server using the Cookie: header. (d) A session cookie is used to maintain a user's session ID. (e) A cookie is sent from the server to the client using the Cookie: header. Answer: (c). (c) is true. The Cookie: header is used to send cookies from the client to the server. The header value is a ;-separated list of name=value cookie pairs. (a) is false. The HttpOnly: attribute species the client accessibility of the cookie value, not its transmission. Specically, if the HttpOnly: attribute is set, then the cookie value will not be accessible using document.cookie in the client. (b) is false. The Secure: attribute species the security of the cookie transmission, not storage. Specically, if the Secure: attribute is set, then the cookie will be sent from the client to the server only for responses resulting from HTTPS requests. (d) is false. A session cookie is commonly thought of as a cookie which disappears once the current browser session is closed. If user sessions are tracked using cookies, then those cookies should be referred to with something like session-id cookies to avoid confusion with session cookies. (e) is false. The Cookie: header is used to send cookies from the client to the server. 7. Which one of the following alternatives describes the largest set of characters all of which are Mustache meta-characters? (a) > and } (b)!, #, /, { and 5

6 (c)! and / (d) #, < and } (e) Answer: (a). (a) is correct because > and } are all among mustache meta-characters: > is used to indicate inclusion of a template using a mustache partial. } is used to close mustache directives. (b) is incorrect because! and are not mustache meta-character. OTOH, #, / and { are all among mustache meta-characters: # is used to start mustache conditional or iterative sections. / is used to close mustache sections. { is used to enclose mustache directives. (c) is incorrect because! is not a mustache meta-character. OTOH, / is among mustache meta-characters: / is used to close mustache sections. (d) is incorrect because < is not a mustache meta-character. OTOH, # and } are all among mustache meta-characters: # is used to start mustache conditional or iterative sections. } is used to close mustache directives. (e) is incorrect because is not a mustache meta-character. 6