The difference between let and Var defined variables in TS and JS

JavaScript Strict mode

The first contact with the Let keyword, there is a very very important concept is "JavaScript strict mode", such as the following code to run the error:

' Hello world. ' ; Console.log (hello);

The error message is as follows:

' Hello world. ' ; ^^ ^syntaxerror:blockconstclass) Not yet supported outside strict mode    ...

The workaround is to add a "JavaScript strict mode" declaration to the file header:

' Use Strict '  'Hello world. ' ; Console.log (hello);

For more detailed instructions on "JavaScript strict mode", please refer to Nanyi's blog
A detailed description of Javascript in strict mode

Let and VAR keywords are not assigned after the declaration of similarities and differences, the same performance

' Use Strict ' ;(function () {  var  vartest;  Let Lettest;   // output undefined  // output undefined} ());

Use undeclared variables to behave differently:

(function () {  // output undefined (note to comment out the following line to run)  //  Direct error: Referenceerror:lettest is not defined  var'test var OK. ' ;   ' test let OK. ' ;} ());

When declaring the same variable repeatedly, it behaves differently:

'Use Strict';(function () {varVartest ='test var OK.'; Let Lettest='test let OK.'; varVartest ='Vartest changed.'; Let Lettest='Lettest changed.';//Direct error: Syntaxerror:identifier ' Lettest ' has already been declaredConsole.log (vartest);//output vartest changed. (Note that you need to comment out the duplicate declaration of the Lettest variable above to run)Console.log (lettest);} ());

Variable action range, different performance

'Use Strict';(function () {varVartest ='test var OK.'; Let Lettest='test let OK.'; {    varVartest ='Vartest changed.'; Let Lettest='Lettest changed.'; } console.log (Vartest); //output "vartest changed.", vartest variable declared in internal "{}" overrides the external Lettest declarationConsole.log (lettest);//output "Test let OK.", Lettest declared in internal "{}" and external lettest are not the same variable}());

Egret under:

Using the undefined let:

class Test {    public name:string"Test";       Public run () {       console.log (a);//The line error block-scoped variable ' a ' used before its declaration.       Console.log (b);         Let A;          var b;     }}

Duplicate define Let:

class Test {    public name:string"Test";       Public Run () {let         A;  The line error  cannot redeclare block-scoped variable ' a '.          var b;                  Let A;          var b;  //The bank error  Cannot redeclare block-scoped variable ' a '.
    }}

Variable range:

classTest { PublicNamestring="Test";  PublicRun () {Let a=1; varb =2; {Let a=2; varb =2;  } console.log (a); //1Console.log (b);//2    }}

 

The difference between let and Var defined variables in TS and JS