【每日一题】- 2020-05-16 - 解析字符串

[azl397985856]

给你一个字符串if (x > 5) { console.log('greater than 5')} else { console.log('less than 5')}

预期：

let a = 1;
const sourceCode = "if (x > 5) { console.log('greater than 5')} else { console.log('less than 5')}";
function eval(text) {
   // do something
}

eval(souceCode) // 'less than 5'
a = 6
eval(souceCode) // 'great than 5'

要求：

• 不能使用eval， script 标签 或者 new Function

[feikerwu]

一个比较挫的想法，放入到script执行🤓🤓🤓

function eval(sourceCode) {
  const reg = /(?<=[\[\(\{\s])x(?=[\[\(\{\s])/
  sourceCode = sourceCode.replace(reg, 'a')
  const script = document.createElement('script')
  script.type = 'text/javascript'
  script.innerHTML = sourceCode
  document.body.appendChild(script)
}

[azl397985856]

@feikerwu 😂 不准用script 标签， 我改下题目描述

[feikerwu]

说下想法，在不用Function，eval, script 的前提下，我们要去执行给定的一段字符，问题转化下其实就是

如何在不用原生提供的js解释器情况下去执行一段js脚本

那很朴素的想法就是，提供方不给执行，我们自己造一个解释器去跑不就好了，至于最后能不能造出来我们另说。所以问题再转化下就是

如何造一个解释器去执行上述提供的字符脚本

解释器就是一段程序，输入是sourcecode字符，程序功能是识别sourcecode内容，并做相应的处理，sourcecode由条件，循环，变量定义，运算等组成，但不管怎么样，这些功能都是可穷举的。 所以第一步就是将sourcecode翻译成上述的功能聚合，以计算机可识别的形式表示，编译原理天然的采用树的结果，或者说AST(Abstract Syntax Tree，抽象语法树)，下面是用 acorn 将上述sourcecode转化出来的语法树表示

{
  "type": "Program",
  "start": 0,
  "end": 78,
  "body": [
    {
      "type": "IfStatement",
      "start": 0,
      "end": 78,
      "test": {
        "type": "BinaryExpression",
        "start": 4,
        "end": 9,
        "left": {
          "type": "Identifier",
          "start": 4,
          "end": 5,
          "name": "x"
        },
        "operator": ">",
        "right": {
          "type": "Literal",
          "start": 8,
          "end": 9,
          "value": 5,
          "raw": "5"
        }
      },
      "consequent": {
        "type": "BlockStatement",
        "start": 11,
        "end": 43,
        "body": [
          {
            "type": "ExpressionStatement",
            "start": 13,
            "end": 42,
            "expression": {
              "type": "CallExpression",
              "start": 13,
              "end": 42,
              "callee": {
                "type": "MemberExpression",
                "start": 13,
                "end": 24,
                "object": {
                  "type": "Identifier",
                  "start": 13,
                  "end": 20,
                  "name": "console"
                },
                "property": {
                  "type": "Identifier",
                  "start": 21,
                  "end": 24,
                  "name": "log"
                },
                "computed": false
              },
              "arguments": [
                {
                  "type": "Literal",
                  "start": 25,
                  "end": 41,
                  "value": "greater than 5",
                  "raw": "'greater than 5'"
                }
              ]
            }
          }
        ]
      },
      "alternate": {
        "type": "BlockStatement",
        "start": 49,
        "end": 78,
        "body": [
          {
            "type": "ExpressionStatement",
            "start": 51,
            "end": 77,
            "expression": {
              "type": "CallExpression",
              "start": 51,
              "end": 77,
              "callee": {
                "type": "MemberExpression",
                "start": 51,
                "end": 62,
                "object": {
                  "type": "Identifier",
                  "start": 51,
                  "end": 58,
                  "name": "console"
                },
                "property": {
                  "type": "Identifier",
                  "start": 59,
                  "end": 62,
                  "name": "log"
                },
                "computed": false
              },
              "arguments": [
                {
                  "type": "Literal",
                  "start": 63,
                  "end": 76,
                  "value": "less than 5",
                  "raw": "'less than 5'"
                }
              ]
            }
          }
        ]
      }
    }
  ],
  "sourceType": "module"
}

可以看到每个节点都有一个 type, 就是上面说的可穷举的程序节点，比如IfStatement(条件)，Literal等等。

第二步，我们拿到上述AST之后，可以给每个type节点加一个处理函数，比如看到ifStatement怎么做，看到 console 的 Identifier 怎么做等等。

定义好每个节点的处理函数之后，从AST root遍历这棵树，对每个节点执行相应的函数

[azl397985856]

@feikerwu 非常棒。 就是这个意思

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