问题
In the following Java snippet, the scope of i is limited to the inside of the for loop. That's why it causes an error. However, in the similar JS snippet, the i is apparently accessible outside of the loop. How is that possible?
Java:
for(int i=0;i<10;i++) {
...
}
System.out.println(i);
Output: "i is not defined"
JS:
for(var i=0;i<10;i++) {
...
}
console.log(i);
Output: 10
I didn't expect to see output from the JS. I want to know how JS differs from Java. How does JavaScript variable scope work?
回答1:
In Javascript "local" variables have function scope, not block scope.
All local variable declarations are "hoisted" to the top of the current scope, so your code is equivalent to:
var i;
for (i = 0; i < 10; ++i) {
}
console.log(i);
Note that while the declaration is hoisted, any assignment is not. e.g. this code
function test() {
console.log(i); // undefined
var i = 1; // declaration and assignment
console.log(i); // 1
}
is equivalent to:
function test() {
var i; // declaration hoisted
console.log(i); // undefined
i = 1; // assignment still happens here
console.log(i); // 1
}
回答2:
Javascript only has two types of scoping - global and functional. Javascript is lexically scoped at the function-level.
来源：https://stackoverflow.com/questions/12645073/java-vs-javascript-variable-scope