js规范–赋值

今天遇到一个小问题：

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”

5. Call PutValue(lref, rval).
6. Return rval.

其实不用看后面，前面就说了，先求左值，再求右值。。