javascript 参考错误：赋值中的左侧无效
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ReferenceError: Invalid left-hand side in assignment
提问by Kevin
my code for a rock paper scissors game (called toss) is as follows:
我的石头剪刀布游戏（称为toss）的代码如下：
var toss = function (one,two) {
if(one = "rock" && two = "rock") {
console.log("Tie! Try again!");
}
// more similar conditions with `else if`
};
When I enter in the parameters
当我输入参数时
toss("rock","rock")
I get this error code:
我收到此错误代码：
"ReferenceError: Invalid left-hand side in assignment"
“参考错误：赋值中的左侧无效”
How to fix it? What this error means and what other cases when this error can happen?
如何解决？此错误意味着什么以及此错误可能发生的其他情况？
回答by Joseph Silber
You have to use ==to compare (or even ===, if you want to compare types). A single =is for assignment.
您必须使用==to compare （或者甚至===，如果您想比较类型）。单个=用于分配。
if (one == 'rock' && two == 'rock') {
console.log('Tie! Try again!');
}
回答by Alexei Levenkov
Common reasons for the error:
错误的常见原因：
- use of assignment (
=) instead of equality (==/===) - assigning to result of function
foo() = 42instead of passing arguments (foo(42)) - simply missing member names (i.e. assuming some default selection) :
getFoo() = 42instead ofgetFoo().theAnswer = 42or array indexinggetArray() = 42instead ofgetArray()[0]= 42
- 使用赋值 (
=) 而不是等式 (==/===) - 分配给函数的结果
foo() = 42而不是传递参数 (foo(42)) - 只是缺少成员名称（即假设一些默认选择）：
getFoo() = 42而不是getFoo().theAnswer = 42或数组索引getArray() = 42而不是getArray()[0]= 42
In this particular case you want to use ==(or better ===- What exactly is Type Coercion in Javascript?) to check for equality (like if(one === "rock" && two === "rock"), but it the actual reason you are getting the error is trickier.
在这种特殊情况下，您想使用==（或更好===- Javascript 中的 Type Coercion 到底是什么？）来检查相等性（例如if(one === "rock" && two === "rock")，但您收到错误的实际原因更棘手。
The reason for the error is Operator precedence. In particular we are looking for &&(precedence 6) and =(precedence 3).
错误的原因是Operator precedence。特别是我们正在寻找&&（优先级 6）和=（优先级 3）。
Let's put braces in the expression according to priority - &&is higher than =so it is executed first similar how one would do 3+4*5+6as 3+(4*5)+6:
在根据优先级的表达，让我们把括号-&&高于=所以它首先执行一个类似的会怎么做3+4*5+6的3+(4*5)+6：
if(one= ("rock" && two) = "rock"){...
Now we have expression similar to multiple assignments like a = b = 42which due to right-to-left associativity executed as a = (b = 42). So adding more braces:
现在我们有类似于多个赋值的表达式，例如a = b = 42由于从右到左的关联性执行为a = (b = 42)。所以添加更多的大括号：
if(one= ( ("rock" && two) = "rock" ) ){...
Finally we arrived to actual problem: ("rock" && two)can't be evaluated to l-value that can be assigned to (in this particular case it will be valueof twoas truthy).
最后，我们到达实际的问题：("rock" && two)无法进行评估，以可分配给L值（在这个特定的情况下，将是值的two作为truthy）。
Note that if you'd use braces to match perceived prioritysurrounding each "equality" with braces you get no errors. Obviously that also producing different result than you'd expect - changes value of both variables and than do &&on two strings "rock" && "rock"resulting in "rock"(which in turn is truthy) all the time due to behavior of logial &&:
请注意，如果您使用大括号将围绕每个“相等”的感知优先级与大括号相匹配，则不会出现错误。显然，这也产生了与您预期不同的结果 -由于logial && 的行为，更改了两个变量的值，而不是&&在两个字符串上进行操作，"rock" && "rock"从而导致"rock"（这反过来又是真的）：
if((one = "rock") && (two = "rock"))
{
// always executed, both one and two are set to "rock"
...
}
For even more details on the error and other cases when it can happen - see specification:
有关错误和其他可能发生的情况的更多详细信息 - 请参阅规范：
LeftHandSideExpression = AssignmentExpression
...
Throw a SyntaxError exception if the following conditions are all true:
...
IsStrictReference(lref) is true
LeftHandSideExpression = AssignmentExpression
...
如果以下条件全部为真，则抛出 SyntaxError 异常：
...
IsStrictReference(lref) 为真
and The Reference Specification Typeexplaining IsStrictReference:
和解释 IsStrictReference的参考规范类型：
... function calls are permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a reference and there is no provision for a user-defined function to return a reference...
...允许函数调用返回引用。承认这种可能性纯粹是为了宿主对象。本规范定义的内置 ECMAScript 函数没有返回引用，也没有规定用户定义的函数返回引用......
回答by Lukasz Dynowski
The same happened for me with eslintmodule. EsLinter throw Parsing error: Invalid left-hand side in assignment expressionfor await in second if statement.
我的eslint模块也发生了同样的情况。EsLinterParsing error: Invalid left-hand side in assignment expression在第二个 if 语句中抛出等待。
if (condition_one) {
let result = await myFunction()
}
if (condition_two) {
let result = await myFunction() // eslint parsing error
}
As strange as it sounds what fixed this error was to add ; semicolonat the end of line where await occurred.
听起来很奇怪，修复此错误的方法是添加; 在发生 await 的行末尾的分号。
if (condition_one) {
let result = await myFunction();
}
if (condition_two) {
let result = await myFunction();
}
