javascript- Uncaught SyntaxError: Identifier * has already been declared

问题

console.log(a) //output:ƒ 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

 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

 var a = 1;
 if(true) {
   function a(){};
   a = 10;
 }
 console.log(a) //output:ƒ 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.

  var a = 1;
  if(true) {
    var a = function(){ console.log() }
    a = 10;
  }
  console.log(a) //output:10

回答1:

This is surprising as javascript var doesn't respect block scope but functional scope...

Sure, but you didn't use var for the declaration of a in the block scope. You used a function declaration, which does respect block scopes (otherwise it would be completely invalid code, as in ES5 strict mode).

It's permissible in javascript to declare same variable twice in the same scope with var as below

Same applies here. The function declaration in the block uses ES6 declaration semantics (like let or const), which does not allow redeclarations.

回答2:

Case 1

console.log(a) //output:ƒ a(){}
var a = 1;
function a(){};
var a = 10;
console.log(a) //output:10

Will be rendered as

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

var a = 1;
if(true){
   function a(){};
   var a = 10;
}
console.log(a)

Will be rendered as

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

var a = 1;
if(true){
    function a(){};
    a = 10;
 }
console.log(a)

Will be rendered as

var a;
a = 1;
if(true) {
    a = function() {};
    let a;
    a = 10;
}
console.log(a); // output : f a(){}

Case 4

var a = 1;
if(true){
    var a= function(){console.log()}
    a = 10;
}
console.log(a)

Will be rendered as

var a;
a = 1;
if(true) {
    a = function(){console.log()}
    a = 10;
}
console.log(a) // output:10

Case 5

var a = 1;
if(true){
    function a(){};
    a = 10;
    console.log(a) 
}
console.log(a)

Will be rendered as

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3:

The simple solution to this is to use 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" })();

来源：https://stackoverflow.com/questions/49774769/javascript-uncaught-syntaxerror-identifier-has-already-been-declared

标签

javascript

scope

var

hoisting