当var x时，x = [x] && x.push（x）之间的javascript数组混淆;

Question

Why does case 1 give us :error: TypeError: x is undefined on line...

 //case 1 var x; x.push(x); console.log(x); 

Why does case 2 shows us Array[undefined] in console

 //case 2 var x; var x = [x]; console.log(x); 

What is the difference between this 2 cases ?

Answer 1

In case one, the line x.push(x); will throw an error if x is not defined - you can't access properties of an undefined object, of course.

In case two, x is not defined at the point the line x = [x]; is run, but you aren't trying to access any properties of x or anything like that - it's just an undefined value, which is OK. So, the expression [x] results in an array with one item, x (which is undefined ), thus [undefined] .

Answer 2

To be able to push anything values to x , you must first say that it is an array like this

var x = [];

About the variable is showing as undefined because any variable in javascript when no values is assigned to it is by default undefined.

Answer 3

Case 1

You are trying to invoke .push() on undefined

var x; // x is declared but value is undefined
x.push(x); // .push() can be invoked only on array types - that's why you get the error
console.log(x); // error happens before this line

Case 2

unndefined is a value in JavaScript. You are creating an array with one entry - undefined .

var x; // x is undefined
var x = [x]; // x = [undefined] - an array with one entry - undefined
console.log(x); // prints the array

Answer 4

There are somethings need to be explained:

1. When you declare a variable, undefined is the default value is been set. Until you set any other like var x = ''; .
2. [].push() is only available on arrays.

So, in your both cases:

Case: 1

var x; // default value set to = undefined
x.push(x); // here push is undefined because undefined don't have push method.
console.log(x); // it doesn't executes because of breaking line of code above.

Case: 2

//case 2
var x; // default value set to = undefined
var x = [x]; // now x is converted to an array and reassigned to x.
console.log(x); // logs array of one undefined value.