Can JavaScript get confused between local variables?

Question

如果我有一对函数，这两个设置局部变量，例如，变量i共同for循环，一个恰巧，而其他正在运行叫，有没有命名空间混乱的危险吗？

Answer 1

Keep in mind that JavaScript does not have block scope, but only function scope.

In addition, if you have nested loops, there will only be one i variable in the following example:

function myFunction() {
  for (var i = 0; i < 10; i++) {
    for (var i = 0; i < 10; i++) {
      // code here will run 10 times instead of 100 times
    }
  }
  // variable i is still accessible from here
}

Douglas Crockford recommends that the var statements should be the first statements in the function body in Code Conventions for the JavaScript Programming Language :

JavaScript does not have block scope, so defining variables in blocks can confuse programmers who are experienced with other C family languages. Define all variables at the top of the function.

I think he has a point, as you can see in the following example, which will not confuse readers into thinking that the variables i and j are held in the scope of the for loop blocks:

function myFunction() {
  var i, j;    // the scope of the variables is now very clear
  for (i = 0; i < 10; i++) {
    for (j = 0; j < 10; j++) {
      // code here will run 100 times
    }
  }
}

Answer 2

As long as you're using var , like this:

for(var i = 0; i < something; i++)

Then it's local and you're fine, if you don't use var , you have a global variable on your hands, and potential issues. Also if a for loop is nested in another, you should use a different variable name for each loop.

Answer 3

It will be a problem if are you referring to nested loops. Every time you enter the second for loop, the value of i (previously set in the outer for loop) will be reset.