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| Section | Weight | Objectives |
|---|---|---|
| Browser and Events | 17% | - Event handling, propagation, listeners - Browser APIs and developer tools - DOM selection and manipulation |
| Server Side JavaScript | 8% | - Node.js fundamentals and core modules - Package management and CLI tools |
| Testing | 7% | - Unit test structure and effectiveness - Test coverage and improvement |
| Objects, Functions, and Classes | 25% | - ES6 classes, inheritance, modules, decorators - Function types, scope, closures, arrow functions - Object creation, properties, prototypes |
| Debugging and Error Handling | 7% | - Error types and handling strategies - Console usage, breakpoints and debugging techniques |
| Variables, Types, and Collections | 23% | - JSON parsing and manipulation - Variable declaration and scope - Data types, type coercion, truthy/falsy values - Strings, numbers, dates, arrays and methods |
| Asynchronous Programming | 13% | - Event loop and execution flow - Callbacks, promises, async/await |
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質問 # 114
A developer wants to use a module called DataPrettyPrint. This module exports one default functioncalled printDate ().
How can a developer import and use the printDate() function?
正解：C
質問 # 115
Function to test:
01 const sum3 = (arr) => {
02 if (!arr.length) return 0;
03 if (arr.length === 1) return arr[0];
04 if (arr.length === 2) return arr[0] + arr[1];
05 return arr[0] + arr[1] + arr[2];
06 };
Which two assert statements are valid tests for this function?
正解：D、E
解説：
Length 1 → returns arr[0] → 0.
Assertion: 0 === 0 → true.
Also a correct and meaningful test.
Therefore, the valid and logically correct tests here are C and D.
Explanation:
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge:
Evaluate each test:
A). sum3([1, '2'])
Array length is 2 → line 4: arr[0] + arr[1] → 1 + '2' → '12' (string).
Assertion: '12' == 12 is true (type coercion).
However, this "test" is not logically correct: a numeric sum function should not be expected to return '12' as a string. It passes only because of weak equality coercion, so it is not a good/valid test of correct behavior.
B). sum3(['hello', 2, 3, 4])
Length ≥ 3 → line 5: 'hello' + 2 + 3 → 'hello2' + 3 → 'hello23'.
'hello23' === NaN is always false because NaN is never equal to anything, not even itself.
This assertion fails and also misunderstands how NaN comparisons work.
C). sum3([-3, 2])
Length 2 → -3 + 2 = -1.
Assertion: -1 === -1 → true.
This is a correct and meaningful test.
質問 # 116
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：C
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 # 117
Given a value, which three options can a developer use to detect if the value is NaN?
正解：A、B、E
解説：
In JavaScript, NaN has some special properties:
NaN is not equal to anything, including itself.
Equality operators (==, ===) do not work for detecting NaN.
Check each option:
A . value === Number.NaN
Number.NaN is just NaN.
But NaN === NaN is always false.
So this expression is never true and cannot be used to detect NaN.
B . value == NaN
Same issue. NaN == NaN is always false.
So this also never detects NaN.
C . Object.is(value, NaN)
Object.is is a more precise equality comparison method.
It treats NaN as equal to NaN: Object.is(NaN, NaN) is true.
So Object.is(value, NaN) is a valid way to detect NaN.
D . value !== value
Because NaN is the only JavaScript value that is not equal to itself,
value !== value is true only when value is NaN.
This is a classic trick for NaN detection.
E . Number.isNaN(value)
Number.isNaN is the recommended modern way to check if a value is NaN without coercion.
It returns true only if value is actually the number NaN.
Therefore, the three correct ways are:
Object.is(value, NaN)
value !== value
Number.isNaN(value)
質問 # 118
After user acceptance testing, the developer is asked to change the webpage background based on the user's location. It works on the developer's computer but not on the tester's machine.
Which two actions will help determine accurate results?
正解：B、C
解説：
Browsers often cache JavaScript, CSS, and HTML. This means:
The developer may be running with the latest version (often with cache disabled in DevTools).
The tester may still be loading an older, cached version of the page where the background-change logic does not exist.
To ensure they are actually testing the latest deployed code:
Disabling cache (A) in the tester's browser (usually via DevTools → Network → "Disable cache") forces fresh loading of all resources on each refresh.
Clearing cache (C) removes older resources from the browser so the next load fetches fresh files from the server.
Options B and D do not directly help verify the tester is using the latest code:
B checks the developer's own browser, which is not the environment showing the problem.
D suggests reworking the code before confirming whether the tester is actually loading the correct version, which is premature.
So the two actions that help produce accurate testing results are A and C.
質問 # 119
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