A debugging problem caused by JS semicolon_javascript skills

Look at the following code:
var a=textbox1.text;
var b=5;
if(a{
a=b;
}
Look at the effect after execution. No matter what the value of textbox1.text is, the condition will be judged. Why? Have you found anything?
It turns out that when I was writing the code, there was an extra semicolon here in if(aNote: What I just wrote down, When I opened it again, there was no content. So I simplified it a bit. I wrote a general meaning