Javascript Guide: Arrays
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Arrays
Arrays are JavaScript’s ordered lists that can store any data types including Strings, Numbers, and Booleans.
Each item inside of an array is at a numbered position.
Syntax
- An array is represented by square brackets and the content inside.
- Elements within an array should be separated by commas.
let colors =["red","blue","green","yellow"];
Access & Update Elements
To access or change an individual item in an array, we use its numbered position. The positions in an array are zero-indexed, meaning the positions start counting from 0.
Syntax
Follow the name of the array with the index of the item within brackets to be accessed or updated.
console.log(colors[3]);
Output:
yellow
To update an item, set the array and index equal = to the new value.
colors[1]="purple";console.log(colors);
Output:
[ 'red', 'purple', 'green', 'yellow' ]
Arrays with let and const
The contents of an array declared with the keyword const and let can be changed; however the const declared array cannot be reassigned a new array or different value.
const capitals =['Athens','Paris','London','Tokyo'];capitals[1]="Berlin";console.log(capitals);
Output:
[ 'Athens', 'Berlin', 'London', 'Tokyo' ]
When we try to reassign capitals a new value, we get an error.
capitals ="Moscow";
Output:
TypeError: Assignment to constant variable.
Built-in Properties & Methods
Built-in Properties
Properties are used to retrieve certain information about the instance of a data type.
Syntax
To utilize a property, follow the name of the array with a period and the property name.
The length property returns the number of elements in the array capitals.
const capitals =['Athens','Paris','London','Tokyo'];console.log(capitals.length);
Output:
4
Built-in Methods
Methods are called on an array to execute certain tasks like adding and removing elements.
Syntax
To utilize a method, follow the name of the array with a period and the method name.
.push() is a method that adds items to the end of an array.
const seasons =['Winter','Spring','Summer'];seasons.push('Autumn');console.log(seasons);
Output:
[ 'Winter', 'Spring', 'Summer', 'Autumn' ]
.pop() is a method that removes the last element of an array.
const seasons =['Winter','Spring','Summer'];seasons.pop();console.log(seasons);
Output:
[ 'Winter', 'Spring' ]
Both .pop() and .push() mutate or change the array on which they are called.
Arrays and Functions
If an array is mutated within a function, that change will be maintained outside of the function as well.
const names =['John','Maria','Will','Anna'];functionaddName(arr){arr.push('Samuel');}addName(names);console.log(names);
Output:
[ 'John', 'Maria', 'Will', 'Anna', 'Samuel' ]
Nested Arrays
Arrays can be nested or contain other arrays.
Syntax
To access the elements within the nested array, chain more bracket notation with index values.
const pizzaOrders =[['Pepperoni','Coke'],['Margherita','Pepsi']];console.log(pizzaOrders[1]);console.log(pizzaOrders[1][0]);
Output:
[ 'Margherita', 'Pepsi' ]
Margherita
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