10 challenging JavaScript Quiz Questions and Answers with Answer Analysis!

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The following JavaScript questions are designed to be challenging and instructive. If you know exactly how to answer every question, that's fine, but if you've made some mistakes and know why, I think it's better!

Question 1: IIFE, HOF

Do the following snippets call function expressions (IIFE), higher-order functions (HOFs), or neither?

((fn, val) => {
  return fn(val);
})(console.log, 5);

Answer: Output 5

Q2: Which of the following array-to-object methods is more appropriate?

const arr = [1, 2, 3];
//第一种
const a = arr.reduce(
  (acc, el, i) => ({ ...acc, [el]: i }),
  {}
);
//第二种
const b = {};
for (let i = 0; i < arr.length; i++) {
  b[arr[i]] = i;
}

Answer: 第二种

When the b-object is set, b[arr[i]] property is set to the current index for each iteration. W hen setting a, extend the syntax (...) A cc creates a shallow copy of the accumulator object () at each iteration and sets a new property. T his shallow copy is more wasteful than not performing a shallow copy. a You need to construct two intermediate objects before the result is reached, and b does not construct any intermediate objects. As a result, b is set more efficiently.

Question 3

Consider the following function: the functionality of superheroMaker What is output when we pass the following two parameters?

const superheroMaker = a => {
  return a instanceof Function ? a() : a;
};


console.log(superheroMaker(() => 'Batman'));
console.log(superheroMaker('Superman'));

Answer: Output "Batman" "Superman"

When passing () => 'Batman' to superheroMaker a is the instance Function T herefore, the function is called and the string "Batman" W hen "Superman" to superheroMaker a it is not a Function so "Superman" only strings. Therefore, the outputs are "Batman" and "Superman".

Q4: Is Object.keys equal to Object.values?

Object Keys, Object Values
const obj = {
  1: 1,
  2: 2,
  3: 3
};


console.log(Object.keys(obj) == Object.values(obj));

Answer: Output false

In this case, Object.keys the key to the string ["1", "2", "3"] but Object.values returns: [1, 2, 3] Even if the values are of the same type, they are not the same object, so equal comparisons return false .

Q5: Basic recursive inspection?

Consider the following recursive functions. If you pass the string "Hello World" to it, what is the output?

const myFunc = str => {
  if (str.length > 1) {
    return myFunc(str.slice(1));
  }


  return str;
};


console.log(myFunc('Hello world'));

Answer: Output "d"

The first time the function is called, str.length is greater than 1 ("Hello World" is 11 characters), so we return the same function called, str.slice(1), which is string "ello World". W e repeat this procedure until the string is only one character long: the character "d", which returns to the initial call to myFunc. Then, we record the character.

Question 6: Functions are equal

What does the following code output?

const a = c => c;
const b = c => c;


console.log(a == b);
console.log(a(7) === b(7));

Answer: Output false,true

In the first test, a and b are different objects in memory; T herefore, a is not equal to b. I n the second test, a (7) returns the number 7 and b (7) returns number 7. These original types are strictly equal to each other.

In this case, it doesn't matter what the equal (-) and identity-comparison operators are;

Question 7: Object properties are equal

a and b have the same different object firstName properties. Are these properties strictly equal to each other?

const a = {
  firstName: 'Bill'
};


const b = {
  firstName: 'Bill'
};


console.log(a.firstName === b.firstName);

Answer: Output true

The answer is yes. a .firstName is the string value "Bill" and b.firstName is the string value "Bill". Two identical strings are always equal.

Question 8: Function function syntax

Suppose myFunc is a function, val1 is a variable, and val2 is a variable. Does JavaScript allow the following syntax?

myFunc(val1)(val2);

Answer: 允许

This is a common pattern of higher-order functions. I f myFunc(val1) returns a function, the function calls val2 as an argument. This is a practical example that you can try:

const timesTable = num1 => {
  return num2 => {
    return num1 * num2;
  };
};


console.log(timesTable(4)(5));
// 20

Question 9: Object property mutation

const a = { firstName: 'Joe' };
const b = a;
b.firstName = 'Pete';
console.log(a);

Answer: Output { firstName: 'Pete' }

When we set up in the second line, b and a point to the same object in memory. Therefore, firstName, b Changing the property to on will change the property of the unique object in firstName memory, so a.firstName will reflect this change.

Question 10: The maximum number in the array

Will the following function always return the maximum number in the array?

function greatestNumberInArray(arr) {
  let greatest = 0;
  for (let i = 0; i < arr.length; i++) {
    if (greatest < arr[i]) {
      greatest = arr[i];
    }
  }
  return greatest;
}

Answer: Output 不是

If there is a value greater than 0 in the array, it is correct, and if they are all less than 0, it returns 0

This function is correct for an array with at least one value of 0 greater than or equal to one. However, if all numbers are below, it returns 0

That's what W3Cschool编程狮 has learned about 10 challenging JavaScript quiz questions and answers.