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Here we go then. The top 30 Javascript warmup exercises for interview preparation. We list the first 10 out of 30 questions in this Part.

Questions🤔💭

Here is the complete list algorithms together with detailed explanations:

• 1. Write a function the reverses a string.

Javascript does not have a build in String Builder class so you cannot modify an existing string. What we can do is create a list that we push each character from the original string starting from the end.

Then we use Array Join to combine the characters as the reversed string.

Here is the gist of the code:

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function reverseString(s) {
 // Create the result list
 const result = [];
 // Start from the end of the string and iterate towards the start
 for (let i = s.length-1; i >= 0; i -= 1) {
 // Push the current char in the list
 result.push(s[i]);
 }
 // Combine the result in a string
 return result.join('');
}

// Examples
console.log(reverseString(""))
console.log(reverseString("abc"))
console.log(reverseString("aaabbbcccd"))

• 2. Write a function that filters out numbers from a list.

We can filter the list and remove anything that is not a number. How do we check if a list item is not a number? Well if we use the typeOf operator we can get:

typeof 1 // number

but if the interviewer asks that valid numbers are strings are also allowed we get:

typeof "1" // string

which is not what we need. The solution is to use isNaN function.

However if you noticed (and maybe the interviewer is picky) there are two cases where this thing fails:

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isNaN('') //false
isNaN(true) //false

So we want to add two more checks for empty string and boolean:

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function isBoolean(value) {
 return typeof value === 'boolean';
}

function isEmptyString(value) {
 return typeof value === 'string' && value.trim().length === 0;
}

Here is the gist of the code:

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function filterNumbers(arr) {
 // Create the result list
 const result = arr.filter(function(value, i) {
 // Filter based on the rules for checking the input is number
 if (isNaN(value) || isBoolean(value) || isEmptyString(value)) {
 return false;
 }
 return true;
 });
 // Return numbers only list
 return result;
}

function isBoolean(value) {
 return typeof value === 'boolean';
}

function isEmptyString(value) {
 return typeof value === 'string' && value.trim().length === 0;
}
 console.log(filterNumbers([1, "2", " ", NaN, Number.POSITIVE_INFINITY, 66, "ab1", false]))

• 3. Write a function that finds an element inside an unsorted list.

This is a typical linear search algorithm that takes Θ(n) time to complete. We need to traverse the whole list and compare the search item with the current item:

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function linearSearch(arr, x) {
 let lo = 0;
 let hi = arr.length-1;
 // Iterate from start till end of list
 while (lo <= hi) {
 // If item was found then return index
 if (arr[lo] === x) {
 return lo;
 } else {
 lo += 1
 }
 }
 // Return -1 to denote the item was not found
 return -1;
}

let arr = [1,3,5,7,9,11,14,18,22];
console.info("Item was found at index: " + linearSearch(arr, 22));

• 4. Write a function that showcases the usage of closures.

Please review existing dev.to articles about what is a Closure. They are better at explaining the details.

Here is a simple example:

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function multiplier(first) {
 let a = first;
 return function(b) {
 return a * b;
 };
}

let multiplyBy2 = multiplier(2);
 console.info(multiplyBy2(4));
console.info(multiplyBy2(5));

You should be able to explain where is the closure there

• 5. What is a Promise? Write a function that returns a Promise.

Please review existing dev.to articles regarding what is a Promise. They are better at explaining the details.

Here is a simple example of a Promise:

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const resultPromise = function(idea) {
 return new Promise(function(resolve, reject) {
 if (idea.isGood) {
 resolve(idea);
 } else {
 reject({
 idea: idea,
 reason: "Not Realistic"
 });
 }
 });
};

resultPromise({idea: "Make Gold from Iron", isGood: false})
 .then(function() {
 console.info("I'm Rich!")
 }, function(err) {
 console.info("Rejected as: " + err.reason);
 });

• 6. Write a function that flattens a list of items.

This is a typical interview question. A list of lists can be flattened so that it contains only one level of items. For example: [1, [2,3, [4]]] should flatten into [1,2,3,4].

In order to flatten we need to recurse as we may have a deep hierarchy of lists. First we create the result list. Then we iterate over all the items and check if the item is a list. If it’s not a list we append to the result. Otherwise we call the calling function again but with the contents of the item instead.

Here is the gist of the code:

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function flatten(arr=[]) {
 // Create the result list;
 let result = [];
 for (let item of arr) {
 // If item is an array we concat the contents
 if (Array.isArray(item)) {
 result = result.concat(flatten(item));
 } else {
 result = result.concat(item);
 }
 }
 return result;
}
 console.info(flatten([[1, 2, [3]], 4]));

• 7. Write a function that finds an element inside a sorted list.

The question seeks to test how well can you implement binary search here. So with binary search you find the middle element and then you check if it’s the target element. If it’s less than the target, then we know that it is located in the first half of the input array. If it’s greater, then it’s located in the second right half of the input array.

Here is the complete code:

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function binarySearch(arr, x) {
 let lo = 0;
 let hi = arr.length-1;
 while (lo <= hi) {
 // Find mid element
 let m = Math.floor((lo + hi) / 2);
 // Check if equal to target
 if (arr[m] === x) {
 return m;
 // Reduce array search space by half
 } else if (arr[m] < x) {
 lo = m + 1;
 } else {
 hi = m - 1;
 }
 }
 // Item not found
 return -1;
}

let arr = [1,3,5,7,9,11,14,18,22];
console.info(console.info("Item was found at index: " + binarySearch(arr, 22)));

• 8. Write a function that accepts two numbers a and b and returns both the division of a and b and their modulo of a and b.

This is straightforward. Here we need to return two values:

a / b and a % b.

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function divMod(a, b) {
 // Be careful for division by zero
 if (b !== 0 ) {
 return [a / b, a % b];
 }
 return [0, 0];
}
 console.info(divMod(16, 5));
console.info(divMod(20, 0));

• 9. Write a function that computes the fibonacci number of N.

In the Fibonacci sequence, every element is the sum of the previous two terms. For example, starting from 0 and 1:

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0, 1, 1 , 2, 3, 5 ,8 ...

We can do this using either recursion or just a while loop. With recursion we may fall into the trap and do it like this:

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function fib(n) {
 if (n === 0) {
 return 0;
 } else if (n === 1) {
 return 1;
 } else {
 return fib(n-1) + fib(n-2);
 }
}

And allow the interviewer to ask why is inefficient. Or we can just add a memoization and make it slightly better:

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function memo(func) {
 let cache = {};
 return function (x) {
 if (x in cache) return cache[x];
 return cache[x] = func(x);
 };
};

let fib = memo(function(n) {
 if (n === 0) {
 return 0;
 } else if (n === 1) {
 return 1;
 } else {
 return fib(n-1) + fib(n-2);
 }
});
 console.info(fib(20))

• 10. Write a function that accepts a string and returns a map with the strings character frequency.

To calculate the frequency we need to use a hash-table. Typically we use either an object mapping the keys to values or even more semantically a javascript Map.

We iterate over all the characters of the string and increase their char counter.

Here is the code for it:

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function computeFrequency(s) {
 // Create the freq hashtable
 const freqTable = new Map();
 // for each char in the string
 for (ch of s) {
 // Check if we have seen it already
 if (!freqTable.has(ch)) {
 freqTable.set(ch, 1);
 } else {
 // Just increase the existing entry
 freqTable.set(ch, freqTable.get(ch) + 1);
 }
 }
 // Return result
 return freqTable;
}
 console.info(computeFrequency("abrakatabra"));

What’s next

Stay put for the next part!

Ref: codethat