JS Interview Prep: Recursion

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Recursion in Javascript

This post goes through code samples utilizing recursion. We will state the problems and then provide runnable code snippets that show the answer. The answers will be hidden you can try them on your own and then peek.

Special thanks to the Rithm School free computer science course and their exercises on github


Question 1: Sum all numbers

Write a function called sumRange. It will take a number and return the sum of all numbers from 1 up to the number passed in.

Sample: sumRange(3) returns 6, since 1 + 2 + 3 = 6.

Answer 1 The most important thing we need for recursive solutions is a base case. There needs to be a way of exiting the loop or the function will go on forever. The base case in the below code code is that when the input is 1, return 1. Eventually the function will return 6, the correct answer

Once you understand this code move on to the next problem

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Question 2: Power function

Write a function called power which takes in a base and an exponent. If the exponent is 0, return 1.

Sample:

console.log(power(2, 4)); // 16
console.log(power(2, 3)); // 8
console.log(power(2, 2)); // 4 
console.log(power(2, 1)); // 2
console.log(power(2, 0)); // 1
Answer 2

You can think of it in terms of this example:

2^4 = 2 * 2^3;
2^3 = 2 * 2^2;
2^2 = 2 * 2^1;
2^1 = 2 * 2^0; // once our exponent is 0 we KNOW that the value is always 1!
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Question 3: Calculate factorial

Write a function that returns the factorial of a number. As a quick refresher, a factorial of a number is the result of that number multiplied by the number before it, and the number before that number, and so on, until you reach 1. The factorial of 1 is just 1.

Sample:

factorial(5); // 5 * 4 * 3 * 2 * 1 === 120
Answer 3
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Question 4: Check all values in an array

Write a function called all which accepts an array and a callback and returns true if every value in the array returns true when passed as parameter to the callback function

Sample:

var allAreLessThanSeven = all([1,2,9], function(num){
	return num < 7;
});

console.log(allAreLessThanSeven); // false
Answer 4
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Question 5: Product of an array

Write a function called productOfArray which takes in an array of numbers and returns the product of them all

Sample:

var six = productOfArray([1,2,3]) // 6
var sixty = productOfArray([1,2,3,10]) // 60
Answer 5
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Question 6: Search JS object

Write a function called contains that searches for a value in a nested object. It returns true if the object contains that value.

Sample:

var nestedObject = {
    data: {
        info: {
            stuff: {
                thing: {
                    moreStuff: {
                        magicNumber: 44,
                        something: 'foo2'
                    }
                }
            }
        }
    }
}

let hasIt = contains(nestedObject, 44); // true
let doesntHaveIt = contains(nestedObject, "foo"); // false
Answer 6
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Question 7: Parse a multi-dimensional array

Given a multi-dimensional integer array, return the total number of integers stored inside this array

Sample:

var seven = totalIntegers([[[5], 3], 0, 2, ['foo'], [], [4, [5, 6]]]); // 7
Answer 7
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Question 8:

Write a function that sums squares of numbers in list that may contain more lists

Sample:

var l = [1,2,3]; 
console.log(SumSquares(l)); // 1 + 4 + 9 = 14

l = [[1,2],3]; 
console.log(SumSquares(l)); // 1 + 4 + 9 = 14

l = [[[[[[[[[1]]]]]]]]] 
console.log(SumSquares(l)); // 1 = 1

l = [10,[[10],10],[10]] 
console.log(SumSquares(l)); // 100 + 100 + 100 + 100 = 400
Answer 8
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Question 9:

The function should return an array containing repetitions of the number argument. For instance, replicate(3, 5) should return [5,5,5]. If the times argument is negative, return an empty array.

Sample:

console.log(replicate(3, 5)) // [5, 5, 5]
console.log(replicate(1, 69)) // [69]
console.log(replicate(-2, 6)) // []
Answer 9
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