In qualche modo tutti gli esempi, sebbene funzionino bene, sono troppo complicati:
- Usano
new Array(), che è un overkill (e un overhead) per un semplice array associativo (dizionario AKA).
- I migliori usano
new Object(). Funziona bene, ma perché tutta questa digitazione extra?
Questa domanda è taggata "principiante", quindi rendiamola semplice.
Il modo semplicissimo di usare un dizionario in JavaScript o "Perché JavaScript non ha un oggetto dizionario speciale?":
// create an empty associative array (in JavaScript it is called ... Object)
var dict = {}; // huh? {} is a shortcut for "new Object()"
// add a key named fred with value 42
dict.fred = 42; // we can do that because "fred" is a constant
// and conforms to id rules
// add a key named 2bob2 with value "twins!"
dict["2bob2"] = "twins!"; // we use the subscript notation because
// the key is arbitrary (not id)
// add an arbitrary dynamic key with a dynamic value
var key = ..., // insanely complex calculations for the key
val = ...; // insanely complex calculations for the value
dict[key] = val;
// read value of "fred"
val = dict.fred;
// read value of 2bob2
val = dict["2bob2"];
// read value of our cool secret key
val = dict[key];
Ora cambiamo i valori:
// change the value of fred
dict.fred = "astra";
// the assignment creates and/or replaces key-value pairs
// change value of 2bob2
dict["2bob2"] = [1, 2, 3]; // any legal value can be used
// change value of our secret key
dict[key] = undefined;
// contrary to popular beliefs assigning "undefined" does not remove the key
// go over all keys and values in our dictionary
for (key in dict) {
// for-in loop goes over all properties including inherited properties
// let's use only our own properties
if (dict.hasOwnProperty(key)) {
console.log("key = " + key + ", value = " + dict[key]);
}
}
Anche cancellare valori è facile:
// let's delete fred
delete dict.fred;
// fred is removed, the rest is still intact
// let's delete 2bob2
delete dict["2bob2"];
// let's delete our secret key
delete dict[key];
// now dict is empty
// let's replace it, recreating all original data
dict = {
fred: 42,
"2bob2": "twins!"
// we can't add the original secret key because it was dynamic,
// we can only add static keys
// ...
// oh well
temp1: val
};
// let's rename temp1 into our secret key:
if (key != "temp1") {
dict[key] = dict.temp1; // copy the value
delete dict.temp1; // kill the old key
} else {
// do nothing, we are good ;-)
}