Detailed usage of let variables in js and the difference with var

I haven’t seen the let variable before, I just encountered it, let’s check it out.

No value assigned after declaration, the performance is the same

(function() {
      var varTest;
      let letTest;
      console.log(varTest); //输出undefined
      console.log(letTest); //输出undefined
    }());

Using undeclared variables, the performance is different:

(function() {
  console.log(varTest); //输出undefined(注意要注释掉下面一行才能运行)
  console.log(letTest); //直接报错：ReferenceError: letTest is not defined

  var varTest = 'test var OK.';
  let letTest = 'test let OK.';
}());

When the same variable is declared repeatedly, the performance is different:

(function() {
      "use strict";
      var varTest = 'test var OK.';
      let letTest = 'test let OK.';

      var varTest = 'varTest changed.';
      let letTest = 'letTest changed.'; //直接报错：SyntaxError: Identifier 'letTest' has already been declared

      console.log(varTest); //输出varTest changed.(注意要注释掉上面letTest变量的重复声明才能运行)
      console.log(letTest);
    }());

Variable scope, different performance:

(function() {
  var varTest = 'test var OK.';
  let letTest = 'test let OK.';

  {
    var varTest = 'varTest changed.';
    let letTest = 'letTest changed.';
  }

  console.log(varTest); //输出"varTest changed."，内部"{}"中声明的varTest变量覆盖外部的letTest声明
  console.log(letTest); //输出"test let OK."，内部"{}"中声明的letTest和外部的letTest不是同一个变量
}());

Remarks:

Use the let statement to declare a variable, and the scope of the variable is limited to the declaration it's in the block. You can assign a value to a variable when you declare it, or you can assign a value to the variable later in the script.

Variables declared using let cannot be used before declaration, otherwise an error will occur.

If your variable is not initialized in a let statement, it will automatically be assigned a JavaScript value undefined

Related articles:

How to define variables with let and var in js

The difference between const, var and let in js

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