Top 10 Must Know JavaScript Functions!

Tags: javascript, webdev, programming, beginners
Published: 2021-09-14 09:15:18

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# Top 10 Must-Know JavaScript Functions

Please check out full [article here](https://www.thedailytechtalk.com/top-10-must-know-javascript-functions)

## 1 filter()

This function filters an array based on the condition you provide and it returns a new array which contains items which satisfy those conditions.


`filter()`

```
const temperatures = [10, 2, 30.5, 23, 41, 11.5, 3];

const coldDays = temperatures.filter(dayTemperature => {
    return dayTemperature < 20;
});

console.log("Total cold days in week were: " + coldDays.length); // 4
```

## 2 map()

Function `map()` is a very simple, it loops over an array and convert each item into something else.

```
const readings = [10, 15, 22.5, 11, 21, 6.5, 93];
const correctedReadings = readings.map(reading => reading + 1.5);
console.log(correctedReadings); // gives [11.5, 16.5, 24, 12.5, 22.5, 8, 94.5]
```

## 3 some()

`some()` is very similar to `filter()` , but `some()` returns boolean instead.

```
const animals = [
    {
        name: 'Dog',
        age: 2
    },
    
    {
        name: 'Cat',
        age: 8
    },

     {
        name: 'Sloth',
        age: 6
    },
];

if(animals.some(animal => {
    return animal.age > 4
})) {
    console.log("Found some animals!")
}
```

## 4 every()

`every()`  is also very similar to `some()` , but `every()` true only if every single element in array satisfy our condition.

```
const isBelowThreshold = (currentValue) => currentValue < 40;

const array1 = [1, 30, 39, 29, 10, 13];

console.log(array1.every(isBelowThreshold));   // true
```

## 5 shift()

The `shift()` method removes the first element from an array and returns removed element. This method changes the length of the array. 

```
const items = ['meat', 'carrot', 'ham', 'bread', 'fish'];
items.shift()
console.log(items); // ['carrot', 'ham', 'bread', 'fish']
```

## 6 unshift()

Just like  `shift()` method removes the first element from an array `unshift()` adds it. This method changes the length of the array and returns the new length of the array as result. 

```
const items = ['milk', 'fish'];
items.unshift('cookie')
console.log(items); // ['cookie', 'milk', 'fish']
```

## 7 slice()

The `slice()` method returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array. The original array will not be modified. 

```
let message = "The quick brown fox jumps over the lazy dog";
let startIndex = message.indexOf('brown');
let endIndex = message.indexOf('jumps');
let newMessage = message.slice(startIndex, endIndex);
console.log(newMessage); // "brown fox "
```

## 8 splice()

`splice()` below start at index 2 (the third place, count starts from 0!! ) of the array, and remove one item.
In our array that would mean that "rabbit" got removed. `splice()` will return new array as result.


```
const animals = ['dog', 'cat', 'rabbit', 'shark', 'sloth'];
animals.splice(2, 1);
console.log(animals); // ["dog", "cat", "shark", "sloth"]
```

## 9 includes()

`includes()` will check every item in the array, and check if any of them includes our condition. It will return boolean.

```
const array1 = [1, 2, 3];

console.log(array1.includes(2));
// expected output: true

const pets = ['cat', 'dog', 'bat'];

console.log(pets.includes('cat'));  //  true

console.log(pets.includes('at'));  //  false
```

## 10 reverse()

`reverse()` method reverses an array. Be careful since `reverse()` is destructive which means it changes the original array.

```
const array1 = ['one', 'two', 'three', 'four'];
console.log(array1);  //  ["one", "two", "three", "four"]

const reversed = array1.reverse();
console.log(reversed);  //  ["four", "three", "two", "one"]
```

*I recently started a new blog [TheDailyTechTalk](https://thedailytechtalk.com) where I create free content. If you liked this post and would like to read more posts about JavaScript please check it out 🎉🎉
🥰*

**If you like what I write and want to support me, please follow me on [Twitter](https://twitter.com/TheDailyTechTa1) to learn more about programming and similar topics ❤️❤️**