JavaScript Arrays and Objects cheatsheet

for people like me who cannot remember the basics

Arrays

Basics

• Create an array name myArray: let myArray = [‘one’, 2, true]
• Access array and store in variable: let firstValue = myArray[0]. firstValue has now the value ‘one’
• Create a nested array (arrays within an array): let myNestedArray = [[“first array”, 1], [“second array”, 2]]
• Access nested arrays: let secondFirst= myNestedArray[1][0]. secondFirst has now the value 2.
• Modify specific array index: myArray[0] = 1;

Manipulate Arrays

Add
.push() to the end
.unshift()to the beginning

Remove
.pop()the end
.shift()beginning
.splice(startIndex, how many elements, other values to insert) middle → modifies original array

Copy
.slice(startIndex, stopIndex) up to, not including
let newArr = [… oldArr] (spread operator) → copies entire array, can insert array into another array:

let oldArr = ['hello', 'world'];
let newArr = [1, 2, ...oldArr, 3];
console.log(newArr)
→[ 1, 2, 'hello', 'world', 3 ]

Check
.indexOf(‘name’) returns index or -1 if not found

Objects

Basics

• Objects are key-value pairs.
• Create an object :

let person = {
  name: “Nicole”,
  age: 19
}

• Access object value with key: person.name
• Change object value: person.age = 20;
• Add new key-value pair: person.birthYear = 2002;
• Multiple levels:

let person = {
  name: “Nicole”,
  age: 19,
  "favourite colours": {
    first: "blue",
    second: "pink",
    third: "green"
   }
}

Dot notation
myObject.key = ‘value’;
Multiple levels works the same
myObject.key.key = ‘value’;

Bracket notation
myObject[‘name of key with space’] = ‘light blue’;
Name of key always in quotes
Must be used when key has a space (e.g. ‘favourite colours’)

Method property of an object
An object can also have a method stored inside of it:

let person = {
  name: “Nicole”,
  age: 19,
  introduceSelf: function() { return “Hello, my name is ” + 
  this.name; }
}
console.log(person.introduceSelf());
 → Hello, my name is Nicole

Remove key
delete object.keyToDelete;
with example object from above: delete person.age;

Check if object has certain property
hasOwnProperty()
object.hasOwnProperty(‘property name’);
with example object from above: person.hasOwnProperty(‘name’); → true

in
‘Property name’ in object;
with example object from above: ‘name’ in person; → true

for…in — iterate through object by key

for(let xyz in object){
   // do something with each key
}

with (as expected) example from above:

for(let x in person){
   console.log(person[x]);
}
→  Nicole
   19

Array of all keys
let array = Object.keys(myObject);
with example from above: let array =Object.keys(person);