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질문 # 38
Given the following code:
let x = ('15' + 10) * 2;
What is the value of x?
정답：A
설명：
________________________________________
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge String + Number results in String concatenation When JavaScript evaluates:
'15' + 10
the + operator sees a string on the left ('15') and a number on the right (10).
In JavaScript, when one operand is a string, the + operator performs string concatenation.
Thus:
'15' + 10 → '1510'
After concatenation, multiplication forces numeric conversion
The expression becomes:
'1510' * 2
The * operator always expects numeric operands.
When given a string, JavaScript attempts numeric conversion using the internal ToNumber operation.
'1510' becomes:
1510
Then:
1510 * 2 = 3020
But 3020 is not in the answer choices-however note the answers represent:
A . 1520
B . 50
C . 35
D . 3020
The correct numeric calculation yields 3020, which matches option D.
But wait-did we misread?
Let's double-check:
('15' + 10) * 2
= '1510' * 2
= 3020
Correct value is 3020.
Therefore the right answer is D.
________________________________________
JavaScript Knowledge Reference (text-only)
The + operator concatenates if either operand is a string.
The * operator always performs numeric coercion.
'numberString' * number converts the string to a number before multiplication.
질문 # 39
Given the following code:
What is the output of line 02?
정답：B
질문 # 40
Refer to the following code (correcting the missing template literal backticks):
let codeName = 'Bond';
let sampleText = `The name is ${codeName}, Jim ${codeName}`;
A developer is trying to determine if a certain substring is part of a string.
Which three code statements return true?
정답：A,C,E
설명：
Comprehensive and Detailed Explanation From Exact Extract JavaScript knowledge:
First, compute sampleText:
let codeName = 'Bond';
let sampleText = `The name is ${codeName}, Jim ${codeName}`;
The template literal evaluates to:
"The name is Bond, Jim Bond"
Now evaluate each statement:
Option A: sampleText.includes('Jim');
String.prototype.includes(substring) returns true if substring occurs anywhere in the string.
sampleText clearly contains "Jim" ("The name is Bond, Jim Bond").
So this returns true.
Option B: sampleText.includes('The', 1);
includes(searchString, position) starts searching from the given position index.
"The name is Bond, Jim Bond" has "The" starting at index 0.
Starting search at index 1 means "The" at index 0 is not considered, and there is no second "The".
So this returns false.
Option C: sampleText.includes('Jim', 4);
"Jim" appears after "The name is Bond, " which is longer than 4 characters; the index of "Jim" is well past 4.
So when searching from index 4, "Jim" is still found.
This returns true.
Option D: sampleText.indexOf('Bond') !== -1;
String.prototype.indexOf(substring) returns:
-1 if the substring is not found,
Otherwise, the starting index of the first occurrence.
"Bond" appears twice in "The name is Bond, Jim Bond".
So sampleText.indexOf('Bond') is some non-negative index (for the first occurrence).
Therefore indexOf('Bond') !== -1 is true.
Option E: sampleText.substring('Jim');
substring expects numeric indexes: substring(startIndex, endIndex?).
If given a string "Jim" as the argument, JavaScript coerces it to a number:
Number('Jim') → NaN
NaN for startIndex is treated as 0.
So sampleText.substring('Jim') is effectively sampleText.substring(0), which returns the full string "The name is Bond, Jim Bond".
This is a string, not a boolean. The question asks "which code statements return true?" This statement returns a string, not the boolean value true.
Thus, the three statements that actually return true (boolean) are:
Study Guide / Concept Reference (no links):
Template literals and ${} interpolation
String.prototype.includes(searchString, position?)
String.prototype.indexOf(substring) and checking for !== -1
String.prototype.substring(start, end?) and argument coercion
Boolean vs non-boolean return types in string methods
________________________________________
질문 # 41
Refer to the code below:
01 new Promise((resolve, reject) => {
02 const fraction = Math.random();
03 if (fraction > 0.5) reject('fraction > 0.5, ' + fraction);
04 resolve(fraction);
05 })
06 .then(() => console.log('resolved'))
07 .catch((error) => console.error(error))
08 .finally(() => console.log('when am I called?'));
When does Promise.finally on line 08 get called?
정답：C
설명：
Comprehensive and Detailed Explanation From Exact Extract JavaScript knowledge:
Behavior of Promise.prototype.finally:
.finally(handler) registers a callback that runs when the promise is settled, meaning:
Either fulfilled (resolved), or
rejected.
Important points:
The finally callback does not receive the promise's value or error (unlike then and catch).
It is executed after the promise is settled, but before the resolution value or rejection reason is passed further down the chain.
It runs in both success and failure paths.
In the given code:
The promise may either:
Call reject('fraction > 0.5, ' + fraction) if fraction > 0.5, or
Call resolve(fraction) otherwise.
In both cases:
If it resolves, .then(() => console.log('resolved')) runs, and then .finally(...) is executed.
If it rejects, .catch((error) => console.error(error)) runs, and then .finally(...) is executed.
So .finally runs:
Not just "when rejected".
Not just "when resolved".
But whenever the promise is resolved or rejected.
Therefore, the correct choice is:
D . When resolved or rejected.
질문 # 42
Refer to the code below:
01 async function functionUnderTest(isOK) {
02 if (isOK) return 'OK';
03 throw new Error('not OK');
04 }
Which assertion accurately tests the above code?
정답：C
설명：
The function:
async function functionUnderTest(isOK) {
if (isOK) return 'OK';
throw new Error('not OK');
}
Behavior:
If isOK is true:
The function resolves (fulfills the promise) with value 'OK'.
If isOK is false:
The function throws, which in an async function becomes a rejected promise with Error('not OK').
We want an assertion that accurately tests the successful path for isOK === true.
Key points about console.assert:
Signature: console.assert(condition, message?)
If condition is falsy, it logs message as an assertion failure.
If condition is truthy, nothing is logged.
We also must understand await:
await functionUnderTest(true) will resolve to the string 'OK'.
Now evaluate options.
Option A:
console.assert(await functionUnderTest(true), 'OK');
await functionUnderTest(true) → 'OK' (truthy).
console.assert('OK', 'OK');
Condition is 'OK' (truthy), so assertion passes.
The message 'OK' is only shown if the condition is falsy, which it is not.
This correctly verifies that the promise resolved (i.e., did not reject). Among the given options, this is the only one that both:
Uses await properly on the async function, and
Associates the message with the expected success "OK".
Option B:
console.assert(await (functionUnderTest(true), 'not OK'));
Inside the parentheses, the comma operator (a, b) evaluates a, discards it, and returns b.
So (functionUnderTest(true), 'not OK'):
Calls functionUnderTest(true) (returns a promise, but its result is discarded), The expression evaluates to the string 'not OK'.
Then await 'not OK' just resolves to 'not OK' immediately (not a promise).
console.assert('not OK');
Condition is 'not OK' (truthy), so the assertion passes regardless of what functionUnderTest actually does.
This does not meaningfully test the function and is misleading.
Option C:
console.assert(functionUnderTest(true), 'OK');
functionUnderTest(true) returns a Promise, not the string 'OK' directly.
A Promise object is always truthy.
So console.assert(promise, 'OK') will pass, even if the promise later rejects.
Also, there is no await, so we are not actually waiting for the async result.
This is not a correct way to assert on an async function's resolved value.
Option D:
console.assert(await functionUnderTest(true), 'not OK');
await functionUnderTest(true) still resolves to 'OK' (truthy).
console.assert('OK', 'not OK'); passes.
However, the message 'not OK' is the opposite of what we expect for the success case and is only used when the condition fails.
This makes the assertion logically inconsistent with the function behavior (it would show "not OK" if the assertion failed).
Therefore, the only assertion that:
Properly waits on the async function, and
Has expectation text that matches the successful behavior ('OK')
is:
Answe r: A
Study Guide / Concept Reference (no links):
async and await behavior in JavaScript
Promises resolving vs rejecting in async functions
console.assert(condition, message) usage
Truthy and falsy values in JavaScript
Comma operator (a, b) semantics
질문 # 43
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