Typescript never gives any error for array & tuples

Question

It never gives any array. Array accept only for one type. but tuples can allow multiple values.

var myTuple = [10,"Hello"];
console.log(myTuple[1]);

var myArray:number[] = [10,20,"Hello"];
console.log(myArray[2]);

[+] Update : my question, why doesn't it give any run time errors ?

It works fine in the typescript playground (i get the error): seems to be a misconfiguration. You'd have to provide your tsconfig.json typescriptlang.org/play?#code/… — flx
– flx, Commented Aug 24, 2021 at 12:05
It doesn't give any runtime errors, because typescript is compiletime only. TS is compiled to javascript, and in javascript there is no such thing as a myArray: number[] — derpirscher
– derpirscher, Commented Aug 24, 2021 at 12:07
@derpirscher Agreed. when i run tsc index.ts then it gives compile error. when i run tsc index.ts | node index.js then it works without having any issues :) — Sameera
– Sameera, Commented Aug 24, 2021 at 12:11

Mahdi Ghajary · Accepted Answer · 2021-08-24 12:22:40Z

2

Typescript is a statically typed language that works compile-time and it eventually converts your code to pure Javascript. the code in your post will be converted to below js code:

var myTuple = [10, "Hello"];
console.log(myTuple[1]);
var myArray = [10, 20, "Hello"];   // notice how number[] is stripped away from your code
console.log(myArray[2]);

Since Javascript is a dynamically typed language, it has no issues with myArray. whereas in Typescript, you have specified before hand that myArray would be a number array type and TS being a statically typed language checks if the value assigned to myArray is indeed a number array or not, which in this case is not. so it raises the following error:

Type 'string' is not assignable to type 'number'.(2322)

answered Aug 24, 2021 at 12:22

Mahdi Ghajary

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