Javascript interview questions set 3

For set 2 please refer to the Javascript interview question set 2.

Let's begin with set 3,

5)Different Array methods?

a)Array map() Method?
.map() It is used when you want to transform elements in an array.
The map() method creates a new array with the results of calling a function for every array element.

The map() method calls the provided function once for each element in an array, in order.

Note: this method does not change the original array.
Ex:var numbers = [65, 44, 12, 4];
var newarray = numbers.map(myFunction)

function myFunction(num) {
return num * 10;
}

b)When to use the filter method?
.filter()  when you want to select a subset of multiple elements from an array.
The filter() method creates an array filled with all array elements that pass a test (provided as a function).
Note: filter() does not change the original array.
Ex:
var ages = [32, 33, 16, 40];

return age >= 18;
}

c)When to use find()?
.find() When you want to select a single element from an array.
The find() method returns the value of the first element in an array that pass a test (provided as a function).
Note: find() does not change the original array.

Ex:
var ages = [3, 10, 18, 20];

return age >= 18;
}

d)When to use reduce() and reduceRight()?
.reduce() when you want to derive a single value from multiple elements in an array
The reduce() method reduces the array to a single value.

The reduce() method executes a provided function for each value of the array (from left-to-right).

The return value of the function is stored in an accumulator (result/total).

Note: this method does not change the original array.
Ex:
var numbers = [175, 50, 25];

numbers.reduce(myFunc);

function myFunc(total, num) {
}

The reduceRight() method reduces the array to a single value.

The reduceRight() method executes a provided function for each value of the array (from right-to-left).

The return value of the function is stored in an accumulator (result/total).
var numbers = [175, 50, 25];

numbers.reduceRight(myFunc);

function myFunc(total, num) {
}

e)When to use every()?
The every() method checks if all elements in an array pass a test (provided as a function).

The every() method executes the function once for each element present in the array:

If it finds an array element where the function returns a false value, every() returns false (and does not check the remaining values)
If no false occur, every() returns true
Note: every() does not change the original array
Ex:
var ages = [32, 33, 16, 40];

return age >= 18;
}

f)Difference between for loop and forEach loop?
When using the .forEach() you pass an individual function with it’s own scope.
In a for loop you’re polluting whatever scope you place the loop in.
Ex of forEach:
var array = [1,2,3];
array.forEach(function(i){
console.log(i);
});

Written By:

Saurabh Joshi

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