The difference between var and let in Javascript

There are three different ways of declaring variables in Javascript: let, var, and const. Let and const behave the same in the lexical environment, but var is very different. Var was introduced in the early days and is generally not used in modern Javascript, but old code is everywhere. This article explains it in detail, hoping not to confuse you.

Example 1 The two are the same

function sayHi() {
  var phrase = "Hello"; // local variable, "var" instead of "let"

  alert(phrase); // Hello
}

sayHi();

alert(phrase); // Error, phrase is not defined

Example 2 The two are different

The following are different, var does not have a block area, only the global or function area, and can cross the block area .

if (true) {
  var test = true; // use "var" instead of "let"
}

alert(test); // true, the variable lives after if

If we use let in the second line, test cannot be accessed in the alter statement. But var ignores the code block, so we actually declare the test variable as a global variable.

The same is true for loop, var cannot be a block area or a local loop area:

for(var i = 0; i < 10; i++) {
  // ...
}

alert(i); // 10, "i" is visible after loop, it&#39;s a global variable

If the code block is in a function, then var becomes a function-level variable:

function sayHi() {
  if (true) {
    var phrase = "Hello";
  }

  alert(phrase); // works
}

sayHi();
alert(phrase); // Error: phrase is not defined

As you can see, var passes through if, for or other code blocks. At this time, the early Javascript block did not have a lexical environment, and var is a legacy product from the early days.

var is processed at the beginning of the function

The var declaration is processed at the beginning of the function (or at the beginning of the script, for global variables). In other words, the var variable is declared at the beginning of the function, no matter where it is defined (assuming it is not defined in a nested function). Code example:
function sayHi() {
phrase = "Hello";

  alert(phrase);

  var phrase;
}

The effect is the same as the following code:

function sayHi() {
  var phrase;

  phrase = "Hello";

  alert(phrase);
}

Even in the ignored code block:

function sayHi() {
  phrase = "Hello"; // (*)

  if (false) {
    var phrase;
  }

  alert(phrase);
}

This behavior is generally called "boost" ” because all vars are hoisted to the top of the function. So, the if(false) code in the example is never executed, but it doesn't matter, var is promoted to the top of the function, and the variable in the (*) line of all examples already exists.

The statement is promoted, but the assignment is not promoted
The demonstration example is as follows:
function sayHi() {
alert(phrase);

  var phrase = "Hello";
}

sayHi();

var = phrase = “Hello” There are two actions
1. Declare variable var
2. Variable assignment=

Declaration is promoted to function top, but the assignment is always at the original place, so the actual effect of the code is as follows:

function sayHi() {
  var phrase; // declaration works at the start...

  alert(phrase); // undefined

  phrase = "Hello"; // ...assignment - when the execution reaches it.
}

sayHi();

Because all all declarations are promoted to the top of the function, we can reference it elsewhere in the function, but the variable is before assignment undefined. The above two example alerts run without errors. Because the variable phrase already exists but has not been assigned a value, undefined is displayed.

Summary

There are two main differences for var:

1. Variables have no block scope, and the minimum visible scope is function level.
2. The variable declaration is promoted to the top of the function.

This difference is not a good thing in most cases. We cannot create block-level variables and occupy more memory; promoting the variable position provides more room for errors. Therefore, new script vars are rarely used.

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