JavaScript Map Example

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.

Map Object

let myMap = new Map()

let keyString = 'a string'
let keyObj    = {}
let keyFunc   = function() {}

// setting the values
myMap.set(keyString, "value associated with 'a string'")
myMap.set(keyObj, 'value associated with keyObj')
myMap.set(keyFunc, 'value associated with keyFunc')

myMap.size              // 3

// getting the values
myMap.get(keyString)    // "value associated with 'a string'"
myMap.get(keyObj)       // "value associated with keyObj"
myMap.get(keyFunc)      // "value associated with keyFunc"

myMap.get('a string')    // "value associated with 'a string'"
                         // because keyString === 'a string'
myMap.get({})            // undefined, because keyObj !== {}
myMap.get(function() {}) // undefined, because keyFunc !== function () {}

Iterating Map

let myMap = new Map()
myMap.set(0, 'zero')
myMap.set(1, 'one')

for (let [key, value] of myMap) {
  console.log(key + ' = ' + value)
}
// 0 = zero
// 1 = one

for (let key of myMap.keys()) {
  console.log(key)
}
// 0
// 1

for (let value of myMap.values()) {
  console.log(value)
}
// zero
// one

for (let [key, value] of myMap.entries()) {
  console.log(key + ' = ' + value)
}
// 0 = zero
// 1 = one

myMap.forEach(function(value, key) {
  console.log(key + ' = ' + value)
})
// 0 = zero
// 1 = one