kode javascript

Cara menggunakan javascript array map performance

Map is a collection of keyed data items, just like an let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235. But the main difference is that let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 allows keys of any type.

Methods and properties are:

let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1237 – creates the map.
let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1238 – stores the value by the key.
let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1239 – returns the value by the key, let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1230 if let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1231 doesn’t exist in map.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1232 – returns let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1233 if the let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1231 exists, let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1235 otherwise.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1236 – removes the element (the key/value pair) by the key.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1237 – removes everything from the map.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1238 – returns the current element count.

For instance:

let map = new Map(); map.set('1', 'str1'); // a string key map.set(1, 'num1'); // a numeric key map.set(true, 'bool1'); // a boolean key // remember the regular Object? it would convert keys to string // Map keeps the type, so these two are different: alert( map.get(1) ); // 'num1' alert( map.get('1') ); // 'str1' alert( map.size ); // 3

As we can see, unlike objects, keys are not converted to strings. Any type of key is possible.

Although let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1239 also works, e.g. we can set map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');2, this is treating map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');3 as a plain JavaScript object, so it implies all corresponding limitations (only string/symbol keys and so on).

So we should use map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');3 methods: map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');5, map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');6 and so on.

Map can also use objects as keys.

For instance:

let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 123

Using objects as keys is one of the most notable and important let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 features. The same does not count for let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235. String as a key in let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235 is fine, but we can’t use another let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235 as a key in let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235.

Let’s try:

As let recipeMap = new Map([ ['cucumber', 500], ['tomatoes', 350], ['onion', 50] ]); // iterate over keys (vegetables) for (let vegetable of recipeMap.keys()) { alert(vegetable); // cucumber, tomatoes, onion } // iterate over values (amounts) for (let amount of recipeMap.values()) { alert(amount); // 500, 350, 50 } // iterate over [key, value] entries for (let entry of recipeMap) { // the same as of recipeMap.entries() alert(entry); // cucumber,500 (and so on) }2 is an object, it converts all let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235 keys, such as let recipeMap = new Map([ ['cucumber', 500], ['tomatoes', 350], ['onion', 50] ]); // iterate over keys (vegetables) for (let vegetable of recipeMap.keys()) { alert(vegetable); // cucumber, tomatoes, onion } // iterate over values (amounts) for (let amount of recipeMap.values()) { alert(amount); // 500, 350, 50 } // iterate over [key, value] entries for (let entry of recipeMap) { // the same as of recipeMap.entries() alert(entry); // cucumber,500 (and so on) }4 and let recipeMap = new Map([ ['cucumber', 500], ['tomatoes', 350], ['onion', 50] ]); // iterate over keys (vegetables) for (let vegetable of recipeMap.keys()) { alert(vegetable); // cucumber, tomatoes, onion } // iterate over values (amounts) for (let amount of recipeMap.values()) { alert(amount); // 500, 350, 50 } // iterate over [key, value] entries for (let entry of recipeMap) { // the same as of recipeMap.entries() alert(entry); // cucumber,500 (and so on) }5 above, to same string let recipeMap = new Map([ ['cucumber', 500], ['tomatoes', 350], ['onion', 50] ]); // iterate over keys (vegetables) for (let vegetable of recipeMap.keys()) { alert(vegetable); // cucumber, tomatoes, onion } // iterate over values (amounts) for (let amount of recipeMap.values()) { alert(amount); // 500, 350, 50 } // iterate over [key, value] entries for (let entry of recipeMap) { // the same as of recipeMap.entries() alert(entry); // cucumber,500 (and so on) }6. Definitely not what we want.

How let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 compares keys

To test keys for equivalence, let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 uses the algorithm . It is roughly the same as strict equality let recipeMap = new Map([ ['cucumber', 500], ['tomatoes', 350], ['onion', 50] ]); // iterate over keys (vegetables) for (let vegetable of recipeMap.keys()) { alert(vegetable); // cucumber, tomatoes, onion } // iterate over values (amounts) for (let amount of recipeMap.values()) { alert(amount); // 500, 350, 50 } // iterate over [key, value] entries for (let entry of recipeMap) { // the same as of recipeMap.entries() alert(entry); // cucumber,500 (and so on) }9, but the difference is that // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });0 is considered equal to // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });0. So // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });0 can be used as the key as well.

This algorithm can’t be changed or customized.

Chaining

Every // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });3 call returns the map itself, so we can “chain” the calls:

map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');

For looping over a map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');3, there are 3 methods:

// runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });5 – returns an iterable for keys,
// runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });6 – returns an iterable for values,
// runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });7 – returns an iterable for entries // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });8, it’s used by default in // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });9.

For instance:

The insertion order is used

The iteration goes in the same order as the values were inserted. let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 preserves this order, unlike a regular let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235.

Besides that, let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 has a built-in // array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str13 method, similar to // array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str14:

// runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });

When a let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 is created, we can pass an array (or another iterable) with key/value pairs for initialization, like this:

// array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str1

If we have a plain object, and we’d like to create a let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 from it, then we can use built-in method Object.entries(obj) that returns an array of key/value pairs for an object exactly in that format.

So we can create a map from an object like this:

let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John

Here, // array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str17 returns the array of key/value pairs: // array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str18. That’s what let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 needs.

We’ve just seen how to create let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 from a plain object with let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John1.

There’s let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John2 method that does the reverse: given an array of // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });8 pairs, it creates an object from them:

let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 2

We can use let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John2 to get a plain object from let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233.

E.g. we store the data in a let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233, but we need to pass it to a 3rd-party code that expects a plain object.

Here we go:

let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 2

A call to // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });7 returns an iterable of key/value pairs, exactly in the right format for let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John2.

We could also make line let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John9 shorter:

That’s the same, because let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John2 expects an iterable object as the argument. Not necessarily an array. And the standard iteration for map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');3 returns same key/value pairs as // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });7. So we get a plain object with same key/values as the map.set('1', 'str1') .set(1, 'num1') .set(true, 'bool1');3.

A let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1234 is a special type collection – “set of values” (without keys), where each value may occur only once.

Its main methods are:

let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 25 – creates the set, and if an let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 26 object is provided (usually an array), copies values from it into the set.
let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 27 – adds a value, returns the set itself.
let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 28 – removes the value, returns let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1233 if let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 20 existed at the moment of the call, otherwise let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1235.
let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 22 – returns let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1233 if the value exists in the set, otherwise let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1235.
let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 25 – removes everything from the set.
let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 26 – is the elements count.

The main feature is that repeated calls of let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 27 with the same value don’t do anything. That’s the reason why each value appears in a let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1234 only once.

For example, we have visitors coming, and we’d like to remember everyone. But repeated visits should not lead to duplicates. A visitor must be “counted” only once.

The alternative to let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1234 could be an array of users, and the code to check for duplicates on every insertion using arr.find. But the performance would be much worse, because this method walks through the whole array checking every element. let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1234 is much better optimized internally for uniqueness checks.

We can loop over a set either with // runs the function for each (key, value) pair recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });9 or using // array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str13:

Note the funny thing. The callback function passed in // array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str13 has 3 arguments: a let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 20, then the same value let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12306, and then the target object. Indeed, the same value appears in the arguments twice.

That’s for compatibility with let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 where the callback passed // array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str13 has three arguments. Looks a bit strange, for sure. But this may help to replace let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 with let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1234 in certain cases with ease, and vice versa.

The same methods let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 has for iterators are also supported:

let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12312 – returns an iterable object for values,
let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12313 – same as let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12312, for compatibility with let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233,
let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12316 – returns an iterable object for entries let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12317, exists for compatibility with let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233.

Methods and properties:

let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12320 – creates the map, with optional let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 26 (e.g. array) of let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12322 pairs for initialization.
let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1238 – stores the value by the key, returns the map itself.
let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1239 – returns the value by the key, let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1230 if let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1231 doesn’t exist in map.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1232 – returns let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1233 if the let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1231 exists, let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1235 otherwise.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1236 – removes the element by the key, returns let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1233 if let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1231 existed at the moment of the call, otherwise let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1235.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1237 – removes everything from the map.
let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1238 – returns the current element count.

The differences from a regular let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1235:

Any keys, objects can be keys.
Additional convenient methods, the let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 12338 property.

Methods and properties:

let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 25 – creates the set, with optional let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 26 (e.g. array) of values for initialization.
let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 27 – adds a value (does nothing if let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 20 exists), returns the set itself.
let prices = Object.fromEntries([ ['banana', 1], ['orange', 2], ['meat', 4] ]); // now prices = { banana: 1, orange: 2, meat: 4 } alert(prices.orange); // 28 – removes the value, returns let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1233 if let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 20 existed at the moment of the call, otherwise let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1235.
let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 22 – returns let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1233 if the value exists in the set, otherwise let john = { name: "John" }; let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object visitsCountObj[ben] = 234; // try to use ben object as the key visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced // That's what got written! alert( visitsCountObj["[object Object]"] ); // 1235.
let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 25 – removes everything from the set.
let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 26 – is the elements count.

Iteration over let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1233 and let john = { name: "John" }; // for every user, let's store their visits count let visitsCountMap = new Map(); // john is the key for the map visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 1234 is always in the insertion order, so we can’t say that these collections are unordered, but we can’t reorder elements or directly get an element by its number.

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