JavaScript is a loosely typed dynamic, prototypal language with seven primitive types.
Everything else, including functions and arrays, is an object.
primitives:
- Null: null
- Undefined: undefined
- Number: 1, 1.5, -1e4, NaN
- BigInt: 1n, 9007199254740993n
- String: ‘str’, “str”,
str ${var} - Boolean: true, false
- Symbol: Symbol(‘description’), Symbol.for(‘namespace’)
The null primitive is typically used to describe the absence of an object
undefined is the absence of a defined value.
- Any variable initialized without a value will be undefined.
- Any expression that attempts access of a non-existent property on an object will result in undefined.
- A function without a return statement will return undefined.
Number type is double-precision floating-point format.
- It allows both integers and decimals but has an integer range of -253-1 to 253-1.
- The BigInt type has no upper/lower limit on integers.
Strings can be created with single or double quotes, or backticks.
- Strings created with backticks are template strings, these can be multiline and support interpolation
- normal strings can only be concatenated together using the plus (+) operator.
Symbols can be used as unique identifier keys in objects.
- Symbol.for method creates/gets a global symbol.
An object is a set of key value pairs
- values can be any primitive type or an object
- keys are called properties.
- An object with a key holding a value that is another object allows for nested data structures:
const obj = { myKey: { thisIs: 'a nested object' } }
console.log(obj.myKey)A prototype is an implicit reference to another object that is queried in property lookups.
- All Javascript objects have prototypes
- If an object doesn’t have a particular property, the object’s prototype is checked for that property.
- If the object’s prototype does not have that property, the object’s prototype’s prototype is checked and so on. This is how inheritance in JavaScript works