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質問 # 147
A team at Universal Containers works on a big project and uses yarn to manage the project's dependencies.
A developer added a dependency to manipulate dates and pushed the updates to the remote repository. The rest of the team complains that the dependency does not get downloaded when they execute yarn.
What could be the reason for this?
正解：A
解説：
In JavaScript server-side development using Node.js, dependency management is typically handled through package managers such as npm or yarn. These tools categorize installed packages into:
dependencies - required for running the application in any environment
devDependencies - required only during development (testing tools, build tools, documentation generators, etc.) When a package is installed using:
yarn add <package> --dev
it is placed under the "devDependencies" section of package.json.
Behavior of Production Mode
Node.js uses the environment variable:
NODE_ENV=production
When this environment variable is set to production, both npm and Yarn follow the standard Node.js convention and skip installing devDependencies. This is done to optimize production builds and reduce deployment size. This is a known and documented behavior in Node.js package management tools.
Therefore, if:
The developer added the date-manipulation library as a dev dependency, and Other team members execute yarn in an environment where NODE_ENV=production is set, then Yarn will not install that dependency because devDependencies are intentionally excluded in production mode.
This explains the behavior described in the question.
Why the Other Options Are Incorrect
Option A:
"YARN_ENV is set to production" is incorrect because Yarn does not use the variable YARN_ENV for dependency installation behavior. Node.js tools use NODE_ENV, not YARN_ENV.
Option B:
This is incorrect because Yarn automatically writes dependencies into package.json. Unlike older npm versions, there is no need for the --save flag.
Option D:
There is no such option as --add. The correct syntax is simply:
yarn add <package>
Missing an option that does not exist cannot be the cause.
JavaScript Knowledge Reference
Node.js uses the environment variable NODE_ENV to determine production or development mode.
Package managers (npm and Yarn) follow the rule that when NODE_ENV=production, only "dependencies" are installed and "devDependencies" are skipped.
Yarn automatically persists installed packages to package.json without requiring --save.
Yarn uses the command yarn add to add dependencies; there is no --add flag.
質問 # 148
const str = 'Salesforce';
Which two statements result in the word "Sales"?
正解：A、C
解説：
Comprehensive and Detailed
"Salesforce" indices: S(0) a(1) l(2) e(3) s(4) f(5) ...
A / C (substring(0, 5)):
substring(start, end) returns characters from start up to but not including end.
str.substring(0, 5) → characters at indices 0,1,2,3,4 → "Sales".
D (substr(0, 5)):
substr(start, length) returns length characters starting at start.
str.substr(0, 5) → 5 chars from index 0 → "Sales".
B is invalid because s is not defined; it should be 0. So the two correct statements are A and D.
質問 # 149
Refer to the following array:
let arr = [1, 2, 3, 4, 5];
Which two lines of code result in a second array, arr2, created such that arr2 is a reference to arr?
正解：A、C
解説：
The correct answers are C and D.
Arrays in JavaScript are objects. When an array variable is assigned directly to another variable, both variables point to the same array in memory.
Option C is correct:
let arr2 = arr;
This does not create a new array. It creates another reference to the same array.
Example:
arr2.push(6);
console.log(arr);
Output:
[1, 2, 3, 4, 5, 6]
Changing arr2 also affects arr because both variables reference the same array.
Option D is also correct:
let arr2 = arr.sort();
The sort() method sorts the array in place and returns the same array reference. Therefore, arr2 refers to the same array object as arr.
The incorrect options create copies:
let arr2 = arr.slice(0, 5);
creates a shallow copy.
let arr2 = Array.from(arr);
also creates a new shallow copy.
So the two lines that make arr2 reference the original arr are C and D.
質問 # 150
Refer to the code below:
Which replacement for the conditionalstatement on line 02 allows a developer to correctly determine that a specific element, myElement on the page had been clicked?
正解：
解説：
event.target.id =='myElement'
質問 # 151
Which statement allows a developer to update the browser navigation history without a page refresh?
正解：A
解説：
The correct answer is C.
The browser provides the History API through:
window.history
To add a new entry to the browser's session history without refreshing the page, JavaScript uses:
window.history.pushState(state, title, url);
So the valid statement is:
window.history.pushState(newStateObject, '', null);
This updates the browser history stack without forcing a full page reload. It is commonly used in single-page applications when changing views or routes dynamically.
The method accepts three arguments:
window.history.pushState(stateObject, title, url);
stateObject stores custom data associated with the history entry.
title is usually passed as an empty string because many browsers ignore it.
url optionally changes the displayed URL. Passing null means no new URL is provided.
Option A is incorrect because:
window.customHistory
is not the standard browser History API object.
Option B is incorrect because:
createState()
is not a valid History API method.
Option D is incorrect because:
updateState()
is not a valid History API method.
The correct browser API method is:
window.history.pushState()
Therefore, the verified answer is C.
質問 # 152
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