概念

在字典中，存储的是[键，值]对，其中键名是用来查询特定元素的。字典和集合很相似，集合以[值，值]的形式存储元素，字典则是以[键，值]的形式来存储元素。字典也称作映射。

实现字典

function Dictionary() {
    let items = {};
    this.set = function(key, value) {
        items[key] = value;
    };
    this.has = function(key) {
        return items.hasOwnProperty(key);
    };
    this.delete = function(key) {
        if (this.has(key)) {
            delete items[key];
            return true;
        }
        return false;
    };
    this.get = function(key) {
        if (this.has(key)) {
            return items[key];
        }
        return undefined;
    };
    this.values = function() {
        let values = [];
        for (let i = 0, keys = Object.keys(items); i < keys.length; i++) {
            values.push(items[keys[i]]);
        }
        return values;
    };
    this.clear = function() {
        items = {};
    };
    this.size = function() {
        return Object.keys(items).length;
    };
    this.keys = function() {
        return Object.keys(items);
    };
    this.getItems = function() {
        return items;
    };
}