javascript-未捕获的语法错误：标识符 * 已被声明
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javascript- Uncaught SyntaxError: Identifier * has already been declared
提问by venkata
console.log(a) //output:? a(){}
var a = 1;
function a(){};
var a = 10;
console.log(a) //output:10
====================
====================
var a = 1;
if(true){
function a(){};
var a = 10;
}
console.log(a) // this code throws Uncaught SyntaxError: Identifier 'a' has already been declared
both above code snippets are same except the if block.why does the latter throws error when its permissible in javascript to delcare same variable twice in the same scope with var as below
除了 if 块之外，上面的两个代码片段都是相同的。 为什么后者在 javascript 中允许在同一范围内使用 var 在同一范围内两次 delcare 相同变量时抛出错误，如下所示
function a(){};
var a = 10; //no error
Also for a slightly different scenario after removing var from `var a = 10 in the above code ,then it works fine but output is surprising
同样对于在上面的代码中从 `var a = 10 中删除 var 之后的稍微不同的情况，然后它工作正常但输出令人惊讶
var a = 1;
if(true) {
function a(){};
a = 10;
}
console.log(a) //output:? a(){}
I am surprised to see this output as I am expecting 10 ..because two variables declared inside the if block refer to the same variable declared above as javascript var doesnt respect block scope but functional scope...so why not the output for above should be 10? where as the below code outputs 10 as i expected when replaced the function definition with function expression.
我很惊讶地看到这个输出，因为我期待 10 ..因为在 if 块中声明的两个变量引用了上面声明的相同变量，因为 javascript var 不尊重块范围而是功能范围......所以为什么不上面的输出应该是 10？当用函数表达式替换函数定义时，下面的代码输出 10 正如我预期的那样。
var a = 1;
if(true) {
var a = function(){ console.log() }
a = 10;
}
console.log(a) //output:10
采纳答案by Bergi
This is surprising as javascript
vardoesn't respect block scope but functional scope...
这是令人惊讶的，因为 javascript
var不尊重块范围而是功能范围......
Sure, but you didn't use varfor the declaration of ain the block scope. You used a function declaration, which doesrespect block scopes(otherwise it would be completely invalid code, as in ES5 strict mode).
当然，但是您没有在块作用域中使用varfor 的声明a。您使用了一个函数声明，它确实尊重块作用域（否则它将是完全无效的代码，就像在 ES5 严格模式中一样）。
It's permissible in javascript to declare same variable twice in the same scope with
varas below
在 javascript 中允许在相同的范围内声明相同的变量两次，
var如下所示
Same applies here. The functiondeclaration in the block uses ES6 declaration semantics (like letor const), which does not allow redeclarations.
这里同样适用。function块中的声明使用 ES6 声明语义（如let或const），不允许重新声明。
回答by Nikhil Aggarwal
Case 1
情况1
console.log(a) //output:? a(){}
var a = 1;
function a(){};
var a = 10;
console.log(a) //output:10
Will be rendered as
将呈现为
var a;
a = function(){}; // now a holds the value as a function
console.log(a); // output : f a(){}
a = 1; // a is a var that holds value 1
a = 10; // a is a var that holds value 10
console.log(a); // output : 10
Case 2
案例二
var a = 1;
if(true){
function a(){};
var a = 10;
}
console.log(a)
Will be rendered as
将呈现为
var a;
a = 1;
if(true) {
a = function() {};
let a; // The function declaration in the block uses ES6 declaration semantics (like let or const), which does not allow re-declarations.
var a; // throws Uncaught SyntaxError: Identifier 'a' has already been declared
a = 10;
}
console.log(a);
Case 3
案例3
var a = 1;
if(true){
function a(){};
a = 10;
}
console.log(a)
Will be rendered as
将呈现为
var a;
a = 1;
if(true) {
a = function() {};
let a;
a = 10;
}
console.log(a); // output : f a(){}
Case 4
案例四
var a = 1;
if(true){
var a= function(){console.log()}
a = 10;
}
console.log(a)
Will be rendered as
将呈现为
var a;
a = 1;
if(true) {
a = function(){console.log()}
a = 10;
}
console.log(a) // output:10
Case 5
案例5
var a = 1;
if(true){
function a(){};
a = 10;
console.log(a)
}
console.log(a)
Will be rendered as
将呈现为
var a;
a = 1;
if(true){
a = function() {};
let a;
a = 10;
console.log(a); // output:10
}
console.log(a); // output : f a(){}
回答by Mohd Sahil
The simple solution to this is to use IIFE
对此的简单解决方案是使用 IIFE
(function() {
var sahil = {
checkThis: function() {
console.log(this);
function checkOther() {
console.log(this);
}
checkOther(); // checkThis() function called in "global context", will
// return "this" as "window"
}
};
var moo = sahil.checkThis;
moo(); // moo() function called in "global context", will return "this" as "window" })();
