javascript测试.length和.length> 0

Question

I have some text in an object's property. I'm testing to see if the object's property has text in it to display; if it doesn't then I display "-" instead of just a blank. It doesn't seem like there's a difference between:

if (MyObject.SomeText && MyObject.SomeText.length) { ... }

if (MyObject.SomeText && MyObject.SomeText.length > 0) { ... }

Are there any edge cases where one syntax would be preferable to the other?

Answer 1

Are there any edge cases where one syntax would be preferable to the other?

Only edge cases where MyObject.SomeText or MyObject.SomeText.length is not what you expect it to be – for instance:

MyObject = {
    SomeText = {
        length: -42
        // or length: true
    }
};

Answer 2

they give same result. Btw, if its "text", then if (MyObject.SomeText) is enough

Answer 3

No, they are equivalent, if the length equals to 0 then it is valued as false .

(This is possible because JS is not strongly typed, in strongly typed languages length would not be castable as boolean).

Answer 4

In javascript, a number is only considered "falsey" when it is 0. Any other value is "truthy". Therefore, the statements number != 0 (comparison, not identity) and !number are exactly equivalent.

The only way your two statements would differ is if length was something other than a positive number.

Answer 5

It's the same:

Boolean(MyObject.SomeText.length)

• returns true if MyObject.SomeText.length != 0
• returns false if MyObject.SomeText.length == 0

and

Boolean(MyObject.SomeText.length>0)

• returns true if MyObject.SomeText.length > 0
• returns false if MyObject.SomeText.length <= 0

But MyObject.SomeText.length can only be 0 or a positive integrer. So

• If MyObject.SomeText.length == 0,
  • Boolean(MyObject.SomeText.length) returns false because MyObject.SomeText.length == 0
  • Boolean(MyObject.SomeText.length>0) returns false because MyObject.SomeText.length <=0
• If MyObject.SomeText.length > 0,
  • Boolean(MyObject.SomeText.length) returns true because MyObject.SomeText.length != 0
  • Boolean(MyObject.SomeText.length>0) returns true because MyObject.SomeText.length > 0