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Methods like includes indexOf lastIndexOf used on string are also used on arrays to search its items.

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indexOf, lastIndexOf, and includes methods

indexOf method

The indexOf method has its syntax shown below:

Syntax

array.indexOf(item[, from])

The search is from left to right.

item => The item is an element in the array.

from => The from specifies where to start the counting. It ignores the matching item before it.

See the example below:

const numbs = [ 5, 0, 2, 4, 2, 1, 4, 6, 0, 10 ];

console.log( numbs.indexOf(0) ); // 1
console.log( numbs.indexOf(0, 2) ); // 8

lastIndexOf method

The lastIndexOf method has its syntax shown below:

Syntax

array.lastIndexOf(item[, from])

The search is from right to left.

item => The item is an element in the array.

from => The from specifies where to start the counting. It ignores the matching item before it.

See the example below:

const numbs = [ 5, 0, 2, 4, 2, 1, 4, 6, 0, 10 ];

console.log( numbs.indexOf(0) ); // 8
console.log( numbs.indexOf(0, 2) ); // 1

If both the indexOf and lastIndexOf items it returns -1.

const numbs = [ 5, 0, 2, 4, 2, 1, 4, 6, 0, 0 ];

console.log( numbs.indexOf(9) ); // -1
console.log( numbs.lastIndexOf(9) ); // -1

includes method

The includes method has its syntax shown below:

Syntax

array.includes(item)

item => If the item exist it returns true, false otherwise

See the example below:

const numbs = [ 5, 0, 2, 4, 2, 1, 4, 6, 0, 10 ];

console.log( numbs.includes(4) ); // true
console.log( numbs.includes(3) ); // false

includes method can also check if NaN but indexOf/lastIndexOf can't.

See the example below:

const numbs = [ 5, 0, 2, 4, NaN, 1, 4, 6, 0, NaN ];

console.log( numbs.indexOf(NaN, 2) ); // -1
console.log( numbs.lastIndexOf(NaN, 2) ); // -1
console.log( numbs.includes(NaN) ); // true

-1 is returned for both indexof and lastIndexOf because equality === doesn't work for NaN.

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find and findIndex methods

The find method finds an object with a specific condition.

The find method has its syntax shown below:

Syntax

array.find(func)

func => func is a callback function

It returns the first matching item in an array

See the example below:

const numbs = [ 5, 0, 2, 4, 2, 1, 4, 6, 0, 10 ];

const greaterThan3 = num => {
    return num > 3;
};

numbs.find(greaterThan3); // 5 => 5 is greater than 3

findIndex

The findIndex method has its syntax shown below:

array.findIndex(function)

It returns the index of the first matching item in an array.

See the example below:

const numbs = [ 5, 0, 2, 4, 2, 1, 4, 6, 0, 10 ];

const greaterThan3 = num => {
    return num > 3;
};

numbs.findIndex(greaterThan3); // 0 => 5 is at index 0

Since functions are first-class objects, we can have an object in place of the callback function func.

See the syntax below:

array.find(obj)

See the example below:

const names = [ 
    { id: 1, name: "Bello" },
    { id: 2, name: "John" },
    { id: 3, name: "Sarah" }
  ];

const id2 = item => {
    return item.id === 2;
};

const findName = names.find(id2);
findName.name;

See another example but with the findIndex method.

const names = [ 
    { id: 1, name: "Bello" },
    { id: 2, name: "John" },
    { id: 3, name: "Sarah" }
  ];

const id2 = item => {
    return item.id === 2;
};

const findName = names.findIndex(id2);
findName; // 2 =>  returns index of item.id

General syntax of find and findIndex methods

The general syntax is shown below:

arr.find(func(item[, index[, array]]))

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filter method

The syntax is similar to find, but filter returns an array of all matching elements:

The find method has its syntax shown below:

Syntax

array.filter(func)

func => func is a callback function

See the examples below:

const numbs = [ 5, 0, 2, 4, 2, 1, 4, 6, 0, 10 ];

const greaterThan3 = num => {
    return num > 3;
};

numbs.filter(greaterThan3); // [ 5, 4, 4, 6, 10 ]

const names = [ 
    { id: 1, name: "Bello" },
    { id: 2, name: "John" },
    { id: 3, name: "Sarah" }
  ];

const id2 = item => {
    return item.id > 2;
};

const findNames = names.filter(id2);
findNames; // [ { id: 2, name: 'John' }, { id: 3, name: 'Sarah' } ]

The general syntax is shown below:

array.filter(func(item[, index[, array]]))

Happy coding!!!

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