A very simple variable leakage problem in js
var a = [];
for (var i = 0; i < 10; iPP) {
a[i] = function () {
console.log(i);
};
}
a[6](); // 10
variable leakage causes the above a [6] to become 10;
I don"t quite understand why a [6] is 10 here. My wrong thinking is as follows
- a [6] there are no variables here, so the memory leak comes from the loop
In the - loop, each time the current degree is assigned to the corresponding array,
a [0] = function () {console.log (0);};
a [1] = function () {console.log (1);};
.
a [9] = function () {console.log (9);};
the value of I is correct each time. After the assignment, each function outputs a fixed number, and there is no variable
3. May I ask what is wrong with my thinking
this question has been asked badly and answered many times.
I is the global variable function call to get I now I has been looped to 10
the following is a simple example that can be understood as going through two loops
var i = 1;
function a1(){ console.log(i) }
var i = 2;
function a2(){ console.log(i) }
a1();//2
a2();//2
solution to change I into a local variable
es6
{
let i = 1;
function a1(){ console.log(i) }
}
{
let i = 2;
function a2(){ console.log(i) }
}
a1();//1
a2();//2
//es6 let
for(let i=0;i<10;iPP) ...
//or
// es5
for (var i = 0; i < 10; iPP) {
a[i] = (function(i){
return function () {
console.log(i);
};
})(i)
}
the problem stems from the lack of a deep understanding of the function scope (chain). Here is a personal opinion, which I hope can help you:
var a = [];
for (var i = 0; i < 10; iPP) { // iwindow
// i
a[i] = function () {
// ii
// a[6]fori10 10
console.log(i);
};
}
a[6](); // 10
When
executes a [6] (), I has already cycled to 10
closure problem. Students, your foundation should be refueled.
in fact, es6's let doesn't grasp the essence of the problem at all
the key point is that js, as a language that allows side effects, its closure only supports referencing variables in the outer scope, but not "referencing" the value of variables in the outer scope
.
CPP does not have this problem ~
simple closure questions, which I have been asked several times during the interview.
