2、JavaScript 基础：词法结构详解

JavaScript 基础：词法结构详解

1. 字符频率直方图示例

在深入探讨 JavaScript 的词法结构之前，先来看一个有趣的示例：字符频率直方图的计算。这个示例程序可以读取标准输入的文本，计算其中每个字符的出现频率，并以直方图的形式输出结果。

/**
 * This Node program reads text from standard input, computes the frequency
 * of each letter in that text, and displays a histogram of the most
 * frequently used characters. It requires Node 12 or higher to run.
 *
 * In a Unix-type environment you can invoke the program like this:
 *    node charfreq.js < corpus.txt
 */
// This class extends Map so that the get() method returns the specified
// value instead of null when the key is not in the map
class DefaultMap extends Map {
    constructor(defaultValue) {
        super();                          // Invoke superclass constructor