034.2 JavaScript Data Structures (weight: 3) WEB-DEVELOPMENT-ESSENTIALS Practice Questions (Page 1)

Questions 1–5

1. 1foundation · easy

   What happens when you try to access a let variable before its declaration?

   • AIt throws a ReferenceError.
   • BIt returns null.
   • CIt returns undefined.
   • DIt throws a SyntaxError.
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2. 2expert · medium

   A developer is writing a function that checks if a value is a valid number. The value can be a number, a string like "42", or null. Which expression correctly returns true only for numeric values (including numeric strings) and false for null?

   • A`typeof value === 'number'`
   • B`value !== null && !isNaN(value)`
   • C`Number(value) === value`
   • D`!isNaN(value)`
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4. 3application · medium

   A developer is debugging a script that logs a variable before it is declared. The code is: `console.log(x); let x = 5;`. What is the output, and why?

   • AReferenceError, because `let` variables are hoisted but not initialized, creating a temporal dead zone.
   • Bundefined, because `let` variables are hoisted and initialized to undefined.
   • CSyntaxError, because `let` cannot be used after a `console.log` statement.
   • D5, because the declaration is hoisted and the assignment is also hoisted.
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5. 4expert · medium

   A developer is reviewing a script that declares a function and a variable with the same name. The code is: `function foo() { return 1; } var foo = 2; console.log(foo);`. What is the output, and why?

   • AReferenceError, because you cannot have a variable and function with the same name.
   • Bundefined, because the variable is hoisted but not assigned until later.
   • C1, because function declarations take precedence over variable declarations.
   • D2, because the variable declaration overrides the function declaration during hoisting.
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6. 5application · medium

   A developer has an array `const numbers = [1, 2, 3, 4, 5];` and needs to create a new array containing only the even numbers. Which code correctly achieves this without modifying the original array?

   • A`const evens = numbers.splice(1, 2);`
   • B`const evens = numbers.map(n => n % 2 === 0);`
   • C`const evens = numbers.filter(n => n % 2 === 0);`
   • D`const evens = numbers.forEach(n => n % 2 === 0);`
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