Question

I saw many code that began like that

var a=a||{};

Now I know that its check if a exist.

My question is why to check it if its at the first of the code?

I mean the programmer know that a is not exist.

Answer 1

The programmer should know if the variable exists or not.

It’s used to mash different pieces of script together without having to keep track of which part of the script is loaded first. Several scripts will have the same piece of code at the start, so they share the same variable.

For the first script, the variable is declared, and there is no previous value for it, so the {} value is used.

For the following scripts that use it, the var is ignored as the variable is already declared, and the previously set value of the variable is used.

Of course, declaring the same variable in several places has a code smell to it. Preferrably you should keep track of what you are doing so that you only declare it once.