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| Section | Weight | Objectives |
|---|---|---|
| Testing | 7% | - Unit testing concepts - Code coverage and best practices - Test doubles and mocks - Testing frameworks and assertions |
| Objects, Functions, and Classes | 25% | - Object creation, properties, and methods - Closures and execution context - Decorators and advanced function patterns - Class syntax, inheritance, and prototypes - Function definitions, parameters, and scope - Modules and imports/exports |
| Variables, Types, and Collections | 23% | - JSON parsing and usage - Array methods and data manipulation - Working with strings, numbers, dates, and booleans - Truthy and falsy values - Type coercion and conversion rules - Variable declaration and initialization |
| Asynchronous Programming | 13% | - Async/await syntax - Promises and chaining - Callbacks and callback hell - Event loop and micro/macro tasks - Fetch API and HTTP requests |
| Browsers and Events | 17% | - Browser APIs and Web Storage - Window and document object properties - Script loading and execution order - Event handling, propagation, and delegation - Document Object Model (DOM) manipulation |
| Server-Side JavaScript | 8% | - Command-line tools and scripts - Core Node.js modules - Node.js fundamentals and runtime - Module system and require/import |
| Debugging and Error Handling | 7% | - Error types and exception handling - Console methods and logging - Defensive programming practices - Debugging tools and breakpoints |
>> JavaScript-Developer-I技術問題 <<
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質問 # 124
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?
正解：C、D、E
解説：
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:
The answer: A, C, D
Study Guide / Concept References (no links):
* Template literals and ${} interpolation
* String.prototype.includes(searchString, position?)
* String.prototype.indexOf(substring) and checking for !== -1
* String.prototype.substring(start, end?) and argument coercion
* Boolean vs non-boolean return types in string methods
質問 # 125
Universal Containers recently launched its new landing page to host a crowd-funding campaign. The page uses an external library to display some third-party ads. Once the page is fully loaded, it creates more than 50 new HTML items placed randomly inside the DOM, like the one in the code below:
All the elements includes the same ad-library-item class, They are hidden by default, and they are randomly displayed while the user navigates through the page.
正解：C
質問 # 126
Refer to the code below:
let car1 = new Promise((_ ,reject)=> setTimeout(reject,2000,"Car1 crashed in")); let car2 = new Promise(resolve => setTimeout(resolve,1500,"Car2 completed")); let car3 = new Promise(resolve => setTimeout(resolve,3000,"Car3 completed")); Promise.race([car1,car2,car3])
.then(value=>{
let result = `${value} the race.`;
}).catch(err=>{
console.log('Race is cancelled.',err);
});
What is the value of result when promise.race execues?
正解：
解説：
Car2 completed the race.
質問 # 127
developer publishes a new version of a package with new features that do not break backward compatibility. The previous version number was 1.1.3.
Following semantic versioning format, what should the new package version number be?
正解：D
質問 # 128
Refer to the code:
01 const event = new CustomEvent(
02 // Missing code
03 );
04 obj.dispatchEvent(event);
A developer needs to dispatch a custom event called update to send information about recordId.
Which two options can be inserted at line 02?
正解：B、D
解説：
The correct constructor signature for CustomEvent is:
new CustomEvent(eventName, optionsObject)
Where:
* eventName is a string.
* optionsObject may include:
* detail # used to pass custom data
* bubbles
* cancelable, etc.
Example:
new CustomEvent( ' update ' , {
detail: { recordId: ' 123abc ' }
});
Now evaluate each option:
Option A
{ type: ' update ' , recordId: ' 123abc ' }
Incorrect: The constructor requires (eventName, options), not a single object. type is not used this way.
Option B
' update ' , { detail: { recordId: ' 123abc ' } }
Correct format. detail is the proper place for custom event data.
Option C
' update ' , ' 123abc '
Incorrect: The second argument must be an object (options), not a string.
Option D
' update ' , { recordId: ' 123abc ' }
Acceptable because any extra properties on the options object are still allowed, even though best practice is to use detail.
This still creates a valid CustomEvent, and the event will dispatch successfully.
Thus the two correct answers are B and D .
JavaScript Knowledge References (text-only)
* new CustomEvent(name, options) is the required syntax.
* The detail property of the options object is the standard location for custom data.
* The second argument must be an object; other types are invalid.
質問 # 129
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