Javascript 在未定义中“使用严格”

Question

Why in this example:

for(x in json) {
   //something here
}

is my variable x undefined when if place 'use strict'; in my script.

Answer 1

if i remember correctly, x will be undefined when you collection will something like this: [undefined, undefined] .

The array is populated but the element is undefined

Answer 2

x doesn't appear to be defined in the loop so you need to use

for(var x in json) {
   //something here
}

Answer 3

strict mode which is activated by placing 'use strict'; in a scope, prevents you from using variables that have not been declared in your code. This is done to prevent creating global variables which is the default behavior in browsers on non strict mode. so in your case you need to do

for(var x in json) {
  // your code here
}

that way the variable x will be creaed, hoisted and will be ready to be used by the loop in strict mode.

Answer 4

In strict mode, variables always have to be defined before they are assigned or used. This means, you have to write

for (var x in json) ...

or

var x;
for (x in json) ...

What you're doing is allowed without strict mode, but bad habit, because it automatically declares the variable x in global scope . A lot of programmers are doing this wrong, even in some references/tutorials.

Answer 5

Yes, it will give you an error because, in strict mode, we have to declare variables before actually, we start using them.

You can try this.

for(var x in json) {
   //something here
}