Struggling with the -t --target option of the tsc transpiler

[Bob Nedwor]

I can't seem to get the -t / --target option to work. I have tried
 --target 'ES2018'  
 --target ES2018
 --target "ES2018"
  -t 'ES2018'
  -t "ES2018'"
  -t ES2018
and with other values like ES2016.
Each time tsc seems to do nothing other than put the contents of the .ts file into the .js file - without even an error message

consman01@BobsBonoboExtreme:~/tspractice$ tsc --version Version 3.5.3 consman01@BobsBonoboExtreme:~/tspractice$ rm a.js consman01@BobsBonoboExtreme:~/tspractice$ tsc --target 'ES2018' a.ts consman01@BobsBonoboExtreme:~/tspractice$ cat a.js const message = 'hello world'; console.log(message); class Foo {    constructor() {        alert('we are running foo..');    } } let fooImpl = new Foo(); consman01@BobsBonoboExtreme:~/tspractice$ consman01@BobsBonoboExtreme:~/tspractice$ rm a.js consman01@BobsBonoboExtreme:~/tspractice$ tsc a.ts consman01@BobsBonoboExtreme:~/tspractice$ cat a.js var message = 'hello world'; console.log(message); var Foo = /** @class */ (function () {    function Foo() {        alert('we are running foo..');    }    return Foo; }()); var fooImpl = new Foo(); consman01@BobsBonoboExtreme:~/tspractice$

Can someone explain what I am doing wrong?  
Thank you.

[Ron McLeod]

Bob Nedwor wrote:Each time tsc seems to do nothing other than put the contents of the .ts file into the .js file

The language features that you are using with TypeScript may translate directly to ES2018.

Can you show us the contents of the .ts source file and the .js transpiled file?

[Ron McLeod]

It looks like your code is already valid ES2018 code ...


TypeScriptconst message = 'hello world'; console.log(message); class Foo {  constructor() {    alert('we are running foo..');  } } let fooImpl = new Foo();
ES2015 (ES6), ES2018const message = 'hello world'; console.log(message); class Foo {    constructor() {        alert('we are running foo..');    } } let fooImpl = new Foo();
ES3, ES5var message = 'hello world'; console.log(message); var Foo = /** @class */ (function () {    function Foo() {        alert('we are running foo..');    }    return Foo; }()); var fooImpl = new Foo();

[Ron McLeod]

If you use TypeScript features which don't translate to JavaScript features , then you will see a difference.  For example, a type alias:

TypeScriptconst message = 'hello world'; console.log(message); type stringOrNumber = string | number; class Foo {  constructor(value: stringOrNumber) {    alert('we are running foo..' + value);  } } let fooImpl = new Foo('Hey!');
ES2018const message = 'hello world'; console.log(message); class Foo {    constructor(value) {        alert('we are running foo..' + value);    } } let fooImpl = new Foo('Hey!');

[Bob Nedwor]

Thanks, everyone but I thought that the idea of tcs is to transpile TypeScript into Javascript. It works great without the --target option.
The default is ES3.

Try
  tsc --all
to see all the options
-t VERSION, --target VERSION                       Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'.

Here is my .tc file ( other than the empty lines, the same as the output .js file when I use the -- target):
consman01@BobsBonoboExtreme:~/tspractice$ cat a.ts const message: string = 'hello world'; console.log(message); class Foo {    constructor() {        alert('we are running foo..');    } } let fooImpl = new Foo(); consman01@BobsBonoboExtreme:~/tspractice$

Can anyone get it to work with any of the VERSION values? other than the default?

[Ron McLeod]

Bob Nedwor wrote:Thanks, everyone but I thought that the idea of tcs is to transpile TypeScript into Javascript. It works great without the --target option.
The default is ES3.

I think the point you are missing is that the transpiler is generating JavaScript code.  The language features that you are using in your TypeScript code are identical in ES2018, so other than removing blank lines and maybe comments, you will not see any difference.

In my last post I tried to show that when you use something in TypeScript where there is no translatable concept it may just get dropped.  Other features like TypeScript enums for example, will get polyfilled with something functionally equivalent.  For example:

TypeScript const message: string = 'hello world'; console.log(message); enum Fruit {  APPLE, BANANA, CHERRY } class Foo {  constructor() {    alert('we are running foo..');  } } let fooImpl = new Foo();
ES2018const message = 'hello world'; console.log(message); var Fruit; (function (Fruit) {    Fruit[Fruit["APPLE"] = 0] = "APPLE";    Fruit[Fruit["BANANA"] = 1] = "BANANA";    Fruit[Fruit["CHERRY"] = 2] = "CHERRY"; })(Fruit || (Fruit = {})); class Foo {    constructor() {        alert('we are running foo..');    } } let fooImpl = new Foo();

[Bob Nedwor]

Ok thanks.
I think I see what you are saying.

So in some cases, the --target simply works as more of a pre-step?

[Ron McLeod]

--target specifies the version of JavaScript which should be generated by tsc.

If your TypeScript code only includes features which are also included on the version that you are transpiling to, then the generated JavaScript code will basically be identical to the original TypeScript code.  The big advantage in that case would be that if you included type declarations in your TypeScript code, the transpiler can verify these at compile/transpile time.

[Bob Nedwor]

Thanks for explaining.
I think I need more practice with Typescript and I should purchase Vlad Riscutia's book, Programming with Types!

So "let" is a valid key word in ES2018?

[Ron McLeod]

You're welcome

Bob Nedwor wrote:So "let" is a valid key word in ES2018?

Yup - it was included starting with ES6/ES2015.  There is a good explanation in this article: Variables and Constants in ES6.

The specification for ES2018 is detailed in ECMAScript® 2018 Language Specification