i can't assign a variable in if statement

hello I've just started learning java script and i have a question about the if statement why when i assign a variable inside the if i don't need to add var before it and when i assign it outside if i must add var so why i can't add var inside the if () like this

if (var number = 5){
document.write("successful");
}
else{
   document.write("failed");
}

and if i didn't add a var it assign the variable to the value like this

if (number = 5){
document.write("successful");
}
else{
    document.write("failed");
}

i know it's a simple question but it got me confused