js 规范 -- 赋值

发表于 更新于 分类于 本文字数： 141 阅读时长 ≈ 1 分钟

今天遇到一个小问题：

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let id: number = 0;
const arr: number[] = [];
arr[id] = ++id;
console.log(arr); // ?

当时我以为是：[undefined, 1]，而实际是 [1]。
理解原理就是查看文档：

The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as follows:

  1. Let lref be the result of evaluating LeftHandSideExpression.
  2. Let rref be the result of evaluating AssignmentExpression.
  3. Let rval be GetValue(rref).
  4. Throw a SyntaxError exception if the following conditions are all true:
  • Type(lref) is Reference is true
  • IsStrictReference(lref) is true
  • Type(GetBase(lref)) is Environment Record
  • GetReferencedName(lref) is either “eval” or “arguments”
  1. Call PutValue(lref, rval).
  2. Return rval.

其实不用看后面，前面就说了，先求左值，再求右值。。