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Why this code doesn't work?

I have the following app.ts

var a = 1;
var b = 3;
console.log(`Before a = ${a}, b = ${b}`);

[a, b] = [b, a];
console.log(`After a = ${a}, b = ${b}`);

when I try to run it with

node app.ts

I have the next:

[a, b] = [b, a];
^

ReferenceError: Invalid left-hand side in assignment
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:373:25)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Function.Module.runMain (module.js:441:10)
    at startup (node.js:139:18)
    at node.js:974:3

But I expect:

Before a = 1, b = 2
After a = 2, b = 1

TypeScript Version 1.8.10

Why this code doesn't work?

I have the following app.ts

var a = 1;
var b = 3;
console.log(`Before a = ${a}, b = ${b}`);

[a, b] = [b, a];
console.log(`After a = ${a}, b = ${b}`);

when I try to run it with

node app.ts

I have the next:

[a, b] = [b, a];
^

ReferenceError: Invalid left-hand side in assignment
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:373:25)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Function.Module.runMain (module.js:441:10)
    at startup (node.js:139:18)
    at node.js:974:3

But I expect:

Before a = 1, b = 2
After a = 2, b = 1

TypeScript Version 1.8.10

• javascript
• typescript
• destructuring

Share Improve this question Follow asked Sep 29, 2016 at 16:05 venbervenber 6111 gold badge11 silver badge44 bronze badges 1

• 5 "TypeScript Version 1.8.10" — You're running it with node. You're running it directly with node. You aren't transpiling it to JavaScript. It is being treated as JavaScript. – Quentin Commented Sep 29, 2016 at 16:07

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3 Answers 3

Sorted by: Reset to default 11

You need to compile your code first.

tsc app.ts

Then run it with

node app.js

TypeScript is a transpiled language. tsc, the typescript compiler, transpiles the TypeScript source to JavaScript. Nodejs is a JavaScript environment which wraps the V8 virtual machine.

You are running the code with NodeJS. You aren't transpiling it from TypeScript to JavaScript, so you are trying to treat your TypeScript as if it were JavaScript.

That might be OK, since TypeScript is a superset of JavaScript. In this particular case, your code also happens to be valid JS.

The destructuring assignments you are trying to use are an ES6 feature that are only supported in NodeJS from 6.5 onwards.

You need to upgrade NodeJS to a newer version.

Alternatively, transpile the TypeScript to ES5 with tsc app.ts

To make it even easier on yourself and run it one command, you can use the ts-node library. After installing to your project, you can just run the following:

ts-node app.ts

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