Detailed introduction to the concept of 'temporary Zenless Zone Zero' in JS

Temporary dead zone

As long as the let command exists in the block-level scope, the variables declared by it will be "binding" in this area. No longer affected by external influences.

var tmp = 123;

if (true) {
  tmp = 'abc'; // ReferenceError
  let tmp;
}

In the above code, there is a global variable tmp, but let declares a local variable tmp in the block-level scope, resulting in the following Or bind this block-level scope, so before let declares the variable, assigning a value to tmp will report an error.

ES6 clearly stipulates that if there are let and const commands in a block, this block will have a closed effect on the variables declared by these commands from the beginning. area. Any use of these variables before declaration will result in an error.

In short, within the code block, the variable is not available until it is declared using the let command. Grammatically, this is called the "temporary dead zone" (TDZ).

if (true) {
  // TDZ开始
  tmp = 'abc'; // ReferenceError
  console.log(tmp); // ReferenceError

  let tmp; // TDZ结束
  console.log(tmp); // undefined

  tmp = 123;
  console.log(tmp); // 123
}

In the above code, before the let command declares the variable tmp, it belongs to the "dead zone" of the variable tmp.

"Temporary dead zone" also means that typeof is no longer a 100% safe operation.

typeof x; // ReferenceError
let x;

In the above code, the variable x is declared using the let command, so before it is declared, it belongs to the "dead zone" of x. As long as you use An error will be reported when reaching this variable. Therefore, typeof will throw a ReferenceError when run.

For comparison, if a variable is not declared at all, using typeof will not report an error.

typeof undeclared_variable // "undefined"

In the above code, undeclared_variable is a variable name that does not exist, and the result is "undefined". Therefore, before let, the typeof operator was 100% safe and would never report an error. This is no longer true. This design is to help everyone develop good programming habits. Variables must be used after they are declared, otherwise an error will be reported.

Some "dead zones" are hidden and not easy to find.

function bar(x = y, y = 2) {
  return [x, y];
}

bar(); // 报错

In the above code, the reason why an error is reported when calling the bar function (some implementations may not report an error) is because the default value of parameter x is equal to another parameter y, and y has not been declared at this time, which belongs to the "dead zone". If the default value of y is x, no error will be reported because x has been declared at this time.

function bar(x = 2, y = x) {
  return [x, y];
}
bar(); // [2, 2]

In addition, the following code will also report an error, which is different from the behavior of var.

// 不报错
var x = x;

// 报错
let x = x;
// ReferenceError: x is not defined

The error reported by the above code is also due to the temporary dead zone. When using let to declare a variable, an error will be reported as long as the variable is used before the declaration is completed. The above line belongs to this situation. Before the declaration statement of variable x is completed, the value of x is taken, resulting in an error "x is not defined".

ES6 stipulates that variable promotion does not occur in temporary dead zones and let, const statements, mainly to reduce runtime errors and prevent this from being used before the variable is declared. variables, leading to unexpected behavior. Mistakes like this are very common in ES5, and now that this provision is in place, it's easy to avoid them.

In short, the essence of the temporary dead zone is that as soon as you enter the current scope, the variable you want to use already exists, but it cannot be obtained. You can only obtain and obtain it until the line of code that declares the variable appears. Use this variable.

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