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  • 笔记-赋值与运算的顺序

    var i=1;
    i=i++;
    alert(i);

    为什么i=1?

    ------------------------------------------------------------------------------------------------------------------------------

    ECMAScript® Language Specification

    11.13.1 Simple Assignment ( = )

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

    Let lref be the result of evaluating LeftHandSideExpression. Let rref
    be the result of evaluating AssignmentExpression. Let rval be
    GetValue(rref).
     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" Call
    PutValue(lref, rval).
     Return rval. NOTE When an assignment occurs
    within strict mode code, its LeftHandSide must not evaluate to an
    unresolvable reference. If it does a ReferenceError exception is
    thrown upon assignment. The LeftHandSide also may not be a reference
    to a data property with the attribute value {[[Writable]]:false}, to
    an accessor property with the attribute value {[[Set]]:undefined}, nor
    to a non-existent property of an object whose [[Extensible]] internal
    property has the value false. In these cases a TypeError exception is
    thrown.

    注意這兩句:

    Let rval be GetValue(rref).
    Call PutValue(lref, rval).

    也就是說,首先執行 i++,此時 i 被賦值 2,並返回 1,然後再賦值 1 給 i,最終 i 爲 1。

    簡單地說,

    var i = 1;
    i = i++;
    

    等價於

    var i, temp; // temp is "rval", which may not be an actual variable. 
    i = 1;
    temp = i;
    i = i + 1;
    i = temp;
    我的github: https://github.com/moux1024
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  • 原文地址:https://www.cnblogs.com/xd1024/p/4637649.html
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