声明对象的两个语句之间的区别

Question

What is the difference between this statement

var X = X || {};

And this. They do the same thing? There is a performance difference?

var X = typeof X === "undefined" ? {} : X;

Answer 1

They're not the same.

The || will return the object when X is any possible falsy value.
The typeof check will only return {} if is X is undefined .

According to this test , the undefined check is nearly twice as fast. That's probably because no type casting is required.

Answer 2

In this case: var X = X || {} var X = X || {} the X variable will be redefined if it's been declared but is falsy . So var X = 0; X = X || {}; var X = 0; X = X || {}; would overwrite the 0 with an object.