function myFunc() {
  let a = b = 0;
}

myFunc();

The reason for this is that assignment operator or = has right-to-left associativity or evaluation. What this means is that when multiple assignment operators appear in a single expression they evaluated from right to left. So our code becomes likes this.

function myFunc() {
  let a = (b = 0);
}

myFunc();

First, the expression b = 0 evaluated and in this example b is not declared. So, The JS Engine makes a global variable b outside this function after that the return value of the expression b = 0 would be 0 and it's assigned to the new local variable a with a let keyword.

We can solve this problem by declaring the variables first before assigning them with value.

function myFunc() {
  let a,b;
  a = b = 0;
}
myFunc();