𝗟𝗲𝘁'𝘀 𝘂𝗻𝗱𝗲𝗿𝘀𝘁𝗮𝗻𝗱 𝗮𝗯𝗼𝘂𝘁 𝗔𝗰𝗰𝗶𝗱𝗲𝗻𝘁𝗮𝗹 𝗚𝗹𝗼𝗯𝗮𝗹 𝗩𝗮𝗿𝗶𝗮𝗯𝗹𝗲 𝗶𝗻 𝗝𝗮𝘃𝗮𝗦𝗰𝗿𝗶𝗽𝘁

𝗳𝘂𝗻𝗰𝘁𝗶𝗼𝗻 𝗳𝗿𝗶𝗱𝗮𝘆() { 𝗹𝗲𝘁 𝗮 = 𝗯 = 𝟬; }

𝗳𝗿𝗶𝗱𝗮𝘆();

console.log("a: ", a); // 𝗥𝗲𝗳𝗲𝗿𝗲𝗻𝗰𝗲𝗘𝗿𝗿𝗼𝗿: 𝗮 𝗶𝘀 𝗻𝗼𝘁 𝗱𝗲𝗳𝗶𝗻𝗲𝗱 console.log("b: ", b); // 𝗯: 𝟬

𝗦𝗼 𝘆𝗼𝘂 𝗺𝗶𝗴𝗵𝘁 𝗯𝗲 𝘁𝗵𝗶𝗻𝗸𝗶𝗻𝗴 𝘄𝗵𝘆 𝗼𝗻 𝗰𝗼𝗻𝘀𝗼𝗹𝗲.𝗹𝗼𝗴("𝗯: ", 𝗯) 𝗶𝘁 𝗶𝘀 𝗴𝗶𝘃𝗶𝗻𝗴 "𝗯: 𝟬" 𝗶𝗻𝘀𝘁𝗲𝗮𝗱 𝗼𝗳 𝗮 𝗥𝗲𝗳𝗲𝗿𝗲𝗻𝗰𝗲𝗘𝗿𝗿𝗼𝗿❓

Let's see

This statement inside function friday()

let a = b = 0; can be break down into two statements, i.e.

b = 0 // global scope let a = b // friday() scope

𝗛𝗲𝗻𝗰𝗲, 𝗮 𝗶𝘀 𝗻𝗼𝘁 𝗮𝘃𝗮𝗶𝗹𝗮𝗯𝗹𝗲 𝗶𝗻 𝗴𝗹𝗼𝗯𝗮𝗹 𝘀𝗰𝗼𝗽𝗲 𝘄𝗵𝗶𝗹𝗲 𝗯 𝗶𝘀 𝗮𝘃𝗮𝗶𝗹𝗮𝗯𝗹𝗲.

That's why on printing a and b in global scope b is printed but a says that it is not defined. 𝗜𝗻 𝗼𝘁𝗵𝗲𝗿 𝘄𝗼𝗿𝗱𝘀, 𝗯 𝗶𝘀 𝗮 𝗴𝗹𝗼𝗯𝗮𝗹 𝘃𝗮𝗿𝗶𝗮𝗯𝗹𝗲 𝗰𝗿𝗲𝗮𝘁𝗲𝗱 𝗮𝗰𝗰𝗶𝗱𝗲𝗻𝘁𝗮𝗹𝗹𝘆.

To 𝗽𝗿𝗲𝘃𝗲𝗻𝘁 your code from creating global variables accidentally, you can 𝘂𝘀𝗲 𝘀𝘁𝗿𝗶𝗰𝘁 mode.