map method

Using the .map method on an array.

Use an anonymous function

Use an function without a name for short logic of just a few lines and if you do not need to reuse the function.

General format:

ARRAY.map(ITEM => EXPRESSION)

ARRAY.map(function(ITEM) { return EXPRESSION })

Example:

const letters = ["a", "b", "c"]

const upper = letters.map(x => x.toUpperCase())

upper
// [ "A", "B", "C" ]

const numbers = [1, 2, 3]
const squares = numbers.map(x => x**2)
// [1, 4, 9]

Without declaring a variable first.

const squares = [1, 2, 3].map(x => x**2)
// [1, 4, 9]

Use a named function

Define a function first and then use it in your .map transformation.

General format:

ARRAY.map(FUNCTION)

Example:

function upper(item) {
  return item.toUpperCase()
}

const letters = ["a", "b", "c"]

const upper = letters.map(upper)

upper
// [ "A", "B", "C" ]

This is equivalent to the more verbose form:

const upper = letters.map(x => upper(x))

Promises

Calling String.upper() on an array of items in a promise:

Promise.resolve([ 'a', 'b', 'c' ] )
  .then(x => x.toUpperCase())
// ['A', 'B', 'C']

When using .then promise chains with map, be careful about how you pass functions.

In other languages, you could use something like .then(console.log) if there is a single paramter but for JS you need to specify a variable. e.g. .then(x => console.log(x))

Comparison with Functional Programming

In formal Functional Programming languages like with Haskell,

  • You would use map as a function. Functional Progamming has no methods.
  • You would use two variable names instead of overwriting the same one, because all objects would be immutable.

JavaScript is not strict here.

  • You apply .map as a method.
  • You could decide to update an array in place.

Note that const keyword prevents reassignment (using = sign). This freezes strings and numbers, but does not freeze an array.

This is valid JS though, updating an object in place.

const myArray = [1, 2, 3])

myArray.push(4)

But const is still good at stopping your from doing this, because you will get an error that a const variable can’t be reassigned.

const myArray = [1, 2, 3]

myArray = myArray.map(myFunction)